text:
Magic The Gathering: probability of casting a 3 drop on turn 3
So, I have a probability related question.
In MTG, which is the probability of landing a 3-drop on turn 3, but using the correct combination. For example, I want to cast a Geist of Saint Traft on turn 3 (assuming I started), what is the probability (given the assumption that I drop 1 land per turn) of having, by turn 3, one island, one plain and one other land and having the card in my hand ready for casting.
I know that as this is drawing without replacement, so some form of Multivariate Hypergeometric Distribution is needed, but I am a little lost.
Let say I have a 60 cards deck: 10 islands, 10 plains, 4 other lands (none of them giving either blue or white mana), 4 Geist of Saint Traft and 32 other cards.
Now, by turn 3 I would have drawn (assuming 1 draw per turn) 9 cards out of the 60 and for dropping the Geist of Saint Traft I would need at least 1 island, at least 1 plain, at least 1 other land (of the other 22 lands) and at least 1 Geist, so in that case would it be the same to calculate the probability of having by turn 3 none of those and subtract that from one? However, I am not sure how to deal with the case of the third land card I should pick it from 22 (the number of lands less the 2 already picked) or from 24 (the total)?

This could maybe go on [math.SE], but I'll write up an answer anyway and if it's judged to be off topic here, a moderator can migrate it.

While deriving an answer is possible, a 10 million monte carlo simulation would take only a few minutes to code up and would probably be faster to execute even than plugging values into the solved formulas once the true, exact answer is found.

Deck = 10 I, 10 P, 4 L, 4 G, 32 O. Probability space: How many ways can the first 7+3 cards be drawn, in order? Possibly consider the entire deck to avoid dependencies. Now, how many of those are "success" events?

I would challenge the usefulness of the resulting figure, as being able to cast a single card in a single turn will hardly mean anything on it's own. You would have to incorporate playable starting hands if you want to get a useful number, which would be quite an ordeal (especially considering Mulligans, the Scry 1 if you're taking a Mulligan, and the two to three cards you draw until your T3 Main Phase). While I concur that math can help with deck building, this seems like a case where it's not worth the hassle, especially since you're likely not going to make decklist changes based on it.

The thing is, having none of those cards is not the only way you'd be unable to cast the Geist on turn 3. You'd have to account for a whole bunch of possibilities:

Zero islands, zero plains, and zero Geists
Zero islands, one plains, and zero Geists
Zero islands, two plains, and zero Geists
... (up to 9 plains)
One island, zero plains, and zero Geists
... (up to 9 islands)
Zero islands, zero plains, and one Geist
... (up to 4 Geists)
Zero islands, one plains, and one Geist
... (up to 4 Geists)
... (up to 9 plains)
...

You get the picture. The number of combinations explodes.
I'd proceed like this. Group the possible arrangements of the deck which do let you cast the Geist by the number of lands you get in 9 draws, i.e. separately consider exactly 3 lands, exactly 4 lands, exactly 5 lands, and so on up to 8. You could also break these groups down further by the number of Geists you get in those same 9 draws, but it turns out not to matter; you only need the probability of getting at least one Geist. It's straightforward to get a computer to iterate over them and compute each probability using the hypergeometric distribution. This table shows the conditional probability that you have drawn a Geist in 9 cards, given the number of lands in those 9 cards:

Then for each number of lands, you have to calculate the conditional probability that, given that you have drawn that many lands, they include a combination of lands that will allow you to cast a Geist. There are two ways to go:

You can break these combinations down by (effectively) colorless lands vs colored lands, since that lets you immediately rule out any situation that doesn't have at least two colored lands. This table shows the conditional probabilities of getting specific counts of colored and total lands, given an otherwise arbitrary 9-card draw (that is, maybe it contains a Geist, maybe not, we're not worrying about that). E.g. the probability that you get exactly 5 lands, 3 of which are colored and 2 of which are colorless, is 0.160926.

After determining the number of colored lands, you just have to subtract off the probability that all your colored lands are of one type, because that's the only way you can fail to meet the colored mana requirements of the Geist with them. That can be calculated with a single-variable hypergeometric distribution. This table shows the conditional probability, given that you have a certain number of colored lands, of them being either all islands or all plains:

When I subtract these numbers from 1 and multiply by the corresponding figures in the previous table, I get the probabilities of drawing lands that actually let you cast a Geist. Then, I add up the probabilities for each number of total lands and multiply those by the corresponding figures from the first table, and finally sum over all possible numbers of Geists and all possible numbers of lands to get an overall probability of being able to cast the Geist of 0.2760477.
I haven't shown it here but I was also able to run the numbers using fractional arithmetic and it works out to 68014212/246385711
The alternative is to break down the possibilities by the total number of lands and calculate the probabilities for each total number individually using the multivariate hypergeometric distribution. For example, given that you get three lands exactly, here are the conditional probabilities of getting various numbers of plains and islands:

Or for five lands:

Then you can get the computer to select the cells corresponding to land configurations which allow you to cast the Geist, i.e. those which have at least one of each. This is a bit more tedious, but still can be automated with the help of a few tricks. By adding up all the selected numbers in each table, you get the conditional probability that you will be able to cast the Geist given that you draw that many lands. Then you can multiply those probabilities by the a priori probabilities of drawing that many lands and at least 1 Geist, and add them up to get the total probability of drawing a Geist and being able to cast it. I get the same answer of 0.2760477 or 68014212/246385711 by this procedure.

I have a messy Jupyter notebook with the code that computes this which I would be willing to fix up and publish, if people are interested, but it will take some time.

There is a legit mathematical answer to this question, but it's a bit outside my ability. What I can offer you, however, is an example of how Magic players of various levels of mathematical knowledge have attempted to tackle the question.
This article by Frank Karsten is a good start. Doing it mathematically, you get stuff like this:

Eventually, Karsten just gives up and decides to simulate it. A sample of his conclusions for 60-card decks:

To have a good (90%) chance of casting a one-drop (e.g. Duress) on turn 1: 14 sources of its color.
To have a good chance of casting a three-drop with one colored mana symbol (e.g. Thirst for Knowledge) on turn 3: 12 sources.
To have a good chance of casting a three-drop with two of the same colored mana symbol (e.g. Liliana of the Veil) on turn 3: 19 sources.

Unfortunately, this particular analysis doesn't focus too much on multi-color non-land cards (more on multi-color lands, which are invaluable for building competitively powerful decks).
He also recommends looking at preexisting competitive decks as a guide, since players tend to build for consistency.
A 2012-season Delver deck is probably a good guidepost, since it's a deck that played Geist on-curve and as one of its most color-demanding cards.
How many white sources is it packing?
lands – 22:

9 Island
2 Plains
4 Glacial Fortress
3 Moorland Haunt
4 Seachrome Coast

card-filtering — 9 cards (plus the Snapcasters):

2 Thought Scour
3 Gitaxian Probe
4 Ponder

So that's 10 white mana sources, plus 4 card-filtering cards that dig deep (the Ponders). Pretty close to the 12 that Karsten said you want to get a 90% chance of casting a single-color three-drop on turn 3.

27.6% if you go first, or 34.5% if you go second. This does not account for mulligans.
Those numbers were obtained experimentally using the code posted below. There's an error in the math presented below. (David Z's theoretical result matches my experimental result, not my theoretical result which is slightly off.0 I'm leaving it up in the hopes that someone can correct me. Maybe I'm treating dependent events as independent? I intend to revisit this.

In the 9 cards drawn from the 60, there must be at least one of the 4 Geist of Saint Traft.
In the remaining 8 cards of 59, there must be at least one of the 10 Islands.
In the remaining 7 cards of 58, there must be at least one of the 10 Plains.
In the remaining 6 cards of 57, there must be at least one of the 22 remaining lands.

Or in other words,

At least one Geist of 4 in 9 of 60.
At least one Island of 10 in 8 of 59.
At least one Plains of 10 in 7 of 58.
At least one land of 22 in 6 of 57.

Useful "trick":
P(X) = 1 - P(not X)

so
P(at least one X) = 1 - P(no X)

P(at least one Geist of 4 in 9 of 60)
= 1 - P(no Geist of 4 in 9 of 60)
= 1 - (56/60 * 55/59 * 54/58 * 53/57 * 52/56 * 51/55 * 50/54 * 49/53 * 48/52)
Perl: my $P1 = 1 - ( mul map { (60-4-$_+1) / (60-$_+1) } 1..9 );
P(at least one Island of 10 in 8 of 59)
= 1 - P(no Island of 10 in 8 of 59)
= 1 - (49/59 * 48/58 * 47/57 * 46/56 * 45/55 * 44/54 * 43/53 * 42/52)
Perl: my $P2 = 1 - ( mul map { (59-10-$_+1) / (59-$_+1) } 1..8 );
P(at least one Plains of 10 in 7 of 58)
= 1 - P(no Plains of 10 in 7 of 58)
= 1 - (48/58 * 47/57 * 46/56 * 45/55 * 44/54 * 43/53 * 42/52)
Perl: my $P3 = 1 - ( mul map { (58-10-$_+1) / (58-$_+1) } 1..7 );
P(at least one land of 22 in 6 of 57)
= 1 - P(no land of 22 in 6 of 57)
= 1 - (35/57 * 34/56 * 33/55 * 32/54 * 31/53 * 30/52)
Perl: my $P4 = 1 - ( mul map { (57-(9+9+4)-$_+1) / (57-$_+1) } 1..6 );

Then you multiply those four probabilities.

#!/usr/bin/perl

use strict;
use warnings;
use feature qw( say );

use List::Util qw( shuffle );

sub mul { my $acc = 1; $acc *= $_ for @_; $acc }

{
   my $P1 = 1 - ( mul map { (60-4-$_+1) / (60-$_+1) } 1..9 );
   my $P2 = 1 - ( mul map { (59-10-$_+1) / (59-$_+1) } 1..8 );
   my $P3 = 1 - ( mul map { (58-10-$_+1) / (58-$_+1) } 1..7 );
   my $P4 = 1 - ( mul map { (57-(9+9+4)-$_+1) / (57-$_+1) } 1..6 );
   my $P = mul $P1, $P2, $P3, $P4;
   say sprintf "Theoretical: %.2f%%", $P*100;
}

{
   my @reference_deck = ( ("G")x4, ("I")x10, ("P")x10, ("O")x4, ("x")x(60-4-10-10-4) );

   my $trials = 1_000_000;
   my $success = 0;
   for (1..$trials) {
      my %hand;
      ++$hand{$_} for ( shuffle @reference_deck )[0..8];
      next if !$hand{G}--;
      next if !$hand{I}--;
      next if !$hand{P}--;
      next if !$hand{I} && !$hand{P} && !$hand{O};
      ++$success;
   }

   my $P = $success/$trials;
   say sprintf "Experimental: %.2f%%", $P*100;
}

Output:
Theoretical: 28.02%
Experimental: 27.65%

Theoretical: 28.02%
Experimental: 27.74%

Theoretical: 28.02%
Experimental: 27.58%

Theoretical: 28.02%
Experimental: 27.63%

I think the 54% chance of just not drawing a Geist in the first 9 cards is kinda squishing the rest of the probabilities down a bit. Would it be more useful to treat that card as fixed and calculate "How likely am I to be able to cast it on turn 3 if it's in my opening hand?"

@Alex P, Experimentally: 56%.

I bet you're right that it's treating dependent events as independent in some case, though I can't immediately see where the mistake is.

P1 is really P(Geist in 9 of 60), and P2 is really P(Island in 9 of 60 | Geist in 9 of 60). We want P(Island in 9 of 60 ∩ Geist in 9 of 60), which is given by P(Island in 9 of 60 | Geist in 9 of 60) * P(Geist in 9 of 60), so as far as I can tell, the math is correct! But even in that simplified case, I get T:38.8, E:39.2.

Re "We want P(Island in 9 of 60 ∩ Geist in 9 of 60)", I think this is the part that's wrong.

I think I may have an idea about the mistake. When you multiply P1 * P2, it of course relies on the events of having at least one Geist in 9 cards and at least one Island in 8 cards being independent. But they actually consist of subevents "have 1 Geist", ..., "have 4 Geists" and "have 1 Island", ..., "have 8 Islands" respectively, some of which have dependency relationships. For example, your calculation gets cross terms corresponding to events like "have 4 Geists and 7 Islands" which can't occur. Then again, I tried to back this up with my probability tables and couldn't figure it out....


text:
"Wrath of God" -VS- "Chord of Calling" + "Selfless Spirit" sacrificed in response
My opponent plays Wrath of God. In response I use Chord of Calling to get Selfless Spirit, so I can sacrifice Selfless Spirit and all of my creatures become indestructible, preventing the effect of Wrath of God.
My opponent said I can't play this way because according to the stack rules, by the time he plays Wrath of God, Selfless Spirit is not in the battlefield.
Can I prevent my creatures from dying to Wrath of God this way?

Welcome to the site. I edited your question for better readability.

You are correct, you can save your creatures with that play.
The stack works according to the "last in, first out" principle. Whatever spell or ability goes on the stack becomes the top-most object on it, and when the stack starts resolving, the top-most object on the stack will resolve first.
Whenever something resolves, players have the chance to play more spells and abilities. So the line of play goes like this:

Your opponent plays Wrath of God
You play Chord of Calling in response
Both of you pass on playing things, and the top-most object, Chord of Calling, resolves. You get the Selfless Spirit on the battlefield.
You sacrifice Selfless Spirit. Its ability becomes the top-most object on the stack. Wrath of God is still on the stack, at the bottom of the stack.
Selfless Spirit's ability resolves, making all of your creatures indestructible.
Wrath of God resolves, and will not kill any of your creatures.

Yes, you could have made your creatures all indestructible this way. What you did was a clever play that could've sealed your victory in this game.
The notion your opponent expressed is nonsense: it doesn't matter whether Selfless Spirit was there at the time already.
First, to get your head around how the stack works, please read this plain explanation of the stack and priority which helped me a lot when I was new to the game.
How making this play would work
Here's what you'd do, and all the priority & stack interaction that would happen:

Casting: Your opponent casts Wrath of God, putting it on the stack.
Priority pass: He passes priority to you. (He must do this before it resolves.)
Casting: While you have priority, you cast Chord of Calling. The stack now looks like this:

Wrath of God (bottom)
Chord of Calling (top)

Priority pass ×2: You pass priority to him. He does nothing with it. He passes priority back to you.
Stack resolution: Because everyone has chosen to pass priority without doing anything, the top item on the stack is resolved. This is Chord of Calling: you search your library for Selfless Spirit and put it on the battlefield. (It's there now. It doesn't matter if it "wasn't there" before.)
Priority assignment: At the end of resolving a spell or ability, the active player gets priority. (That could be you or your opponent; it's whoever has the current turn.)
Priority pass (maybe) & ability activation: Before Wrath of God can resolve, both players must pass priority again. But when you get priority, you will use it to sacrifice Selfless Spirit (and immediately move it to the graveyard in paying its cost). The stack now looks like this:

Wrath of God (bottom)
Selfless Spirit's ability (top)

Priority pass ×2 & resolution: Both players pass priority again, resolving Selfless Spirit's ability, and giving all your creatures indestructible. Active player gets priority again.
Priority pass ×2 & resolution: Both players pass priority again. Wrath of God attempts to destroy all creatures, but does not destroy your creatures --- it just destroys your opponent's. Nice one.

The stack is now empty and you're both free to do whatever. You have your creatures and your opponent no longer has any of theirs, probably.

Excellent, detailed answer! I wish more "stack/priority" questions got answers that broke down the play into every priority assignment and pass!

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text:
MTG AER: Tezzeret's Touch and Audacious Infiltrator artifact interaction
Tezzeret's Touch specifically indicates that the artifact it enchants is a Creature in addition to other types, rather than Artifact Creature. Does this mean this enchanted artifact can block Audacious Infiltrator as it is not specifically an Artifact Creature or does it become an Artifact Creature by default as it is both an Artifact and a Creature?

'Artifact Creature' is not a thing by itself, it is two separate things. What it means is 'this permanent/card/spell is an Artifact' and 'this permanent/card/spell is a Creature'. Since the permanent is both a Creature and an Artifact it is effected by things that effect Artifact Creatures.

205.2b Some objects have more than one card type (for example, an artifact creature). Such objects satisfy the criteria for any effect that applies to any of their card types.

Tezzeret's touch says "in addition to other types" and since the creature is already an artifact, it becomes a artifact creature (plus anything else, like enchantment if you enchanted Bident of Thassa) so it can't block Audacious Infiltrator.
Tezzeret's touch enchants an artifact so if the enchanted permanent loses the artifact type (e.g. it was targeted by Liquimetal Coating earlier in the turn and it is now at the end of the turn) then Tezzeret's touch would fall off the creature as it now enchanting an illegal permanent. This is why it can't lose the artifact type.

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text:
What happens when I equip Helm of the Host on Gideon?
Say I activate Gideon of the Trials' second ability, and have him become a creature. I then equip him with a Helm of the Host and enter combat. What do I get?
Do I get a 4/4 indestructible token, that's only a creature? One which reverts to being a planeswalker at the end of turn? Or do I get a new planeswalker immediately?
If the copy is a planeswalker, does it gain haste, even though it's not a creature? I assume it wouldn't change anything either way since I wouldn't be able to trigger the copy's second ability during combat, but I'm just curious, though if I manage to get a second combat phase it could be useful.

You will get a non-legendary, non-creature planeswalker. You cannot immediately attack with it, but you get to keep all of them.
If you copy an object, only certain values will be copied, among them its original characteristics and existing copy effects. Other effects, including type-changing effects as is the case here, will not be copied.

706.2. When copying an object, the copy acquires the copiable values of the original object’s characteristics and, for an object on the stack, choices made when casting or activating it (mode, targets, the value of X, whether it was kicked, how it will affect multiple targets, and so on). The “copiable values” are the values derived from the text printed on the object (that text being name, mana cost, color indicator, card type, subtype, supertype, rules text, power, toughness, and/or loyalty), as modified by other copy effects, by its face-down status, and by “as . . . enters the battlefield” and “as . . . is turned face up” abilities that set power and toughness (and may also set additional characteristics). Other effects (including type-changing and text-changing effects), status, and counters are not copied.

Your token planeswalker will have haste, but you only get the token at the beginning of combat, which is no longer your main phase and thus too late to make him a creature through his zero ability. You will have to wait until the next combat phase to attack with the newly created copy. If you somehow get another combat phase during the same turn, you can first activate the copy to make it a creature and, thanks to haste, attack with it.
Due to the recent removal of the planeswalker uniqueness rule, you will also not have to choose which of the planeswalkers to keep. Previously, you could only have 1 planeswalker of each planeswalker type at a time, whereas now planeswalkers are legendary and follow the regular legend rule. Since Helm of the Hosts creates non-legendary copies, the legend rule does not apply.

704.5j If a player controls two or more legendary permanents with the same name, that player chooses one of them, and the rest are put into their owners’ graveyards. This is called the “legend rule.”
Planeswalker Uniqueness Rule (Obsolete)
Older versions of the rules stated that a player who controlled two or more planeswalkers with the same planeswalker type would put all but one of those planeswalkers into their owners’ graveyards. This rule was called the “planeswalker uniqueness rule” and no longer exists.

Finally, when Gideon stops being a creature at the end of turn, Helm of the Hosts will become unattached from him:

704.5n If an Equipment or Fortification is attached to an illegal permanent, it becomes unattached from that permanent. It remains on the battlefield.

Would you be able to attack with the new Gideons if you had a Teferi, Temporal Archmage emblem?

Please ask a new question if you have a new question.

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text:
Assigning damage less than power
Is it possible to assign damage to a single blocking creature that is less than the attacking creature's power? If so, it is possible to assign less than lethal damage?
Suppose I attack with a big 20/20 creature, and my opponent blocks with Saber Ants (Whenever Saber Ants is dealt damage, you may put that many 1/1 green Insect creature tokens onto the battlefield). I don't want to give him 20 tokens, so can I just assign 3 damage and not the rest? Maybe I want to keep his Ants alive for some reason - can I assign less than lethal damage?

The attacker has to assign all combat damage to creatures blocking each of his or her creatures.

510.1c A blocked creature assigns its combat damage to the creatures blocking it. If no creatures are currently blocking it (if, for example, they were destroyed or removed from combat), it assigns no combat damage. If exactly one creature is blocking it, it assigns all its combat damage to that creature. [..]

One notable exception to that rule is if the attacking creature has the trample ability. In that case, you only have to assign lethal damage (3 in your example) to the creature(s) blocking it, and the rest may be assigned to the defending player or the blocking creatures as you want.

702.19b The controller of an attacking creature with trample first assigns damage to the creature(s) blocking it. Once all those blocking creatures are assigned lethal damage, any remaining damage is assigned as its controller chooses among those blocking creatures and the player or planeswalker the creature is attacking. [..]

This is not really an exception. Just as you distribute the damage between the blocking creatures, in this case you distribute the damage between the blockers and their controller. In all this cases you have to assign all the damage.

Note which part I bolded in the 510.1c excerpt. That part of the rule gets modified by trample. That's what I called an exception. I didn't say that trample lets you distribute less than the attack of the attacking creature, it lets you distribute less than its attack to creatures blocking it. Which was the core of OP's question, how do I deal less than full damage to a single blocker.

I understand your intent now. I was looking at the case from the perspective of a question "Can I assign less damage than I have power?" not from the perspective of a question "Can I assign less damage than I have power to a blocking creature?"

If a 2/2 creature is blocked by 2 1/1's could that creature choose to assign all damage to one of them?

@Dennis van Opstal Yes, that came up a lot in Shadows Over Innistrad limited with the Devil tokens (1/1, when this creature dies deal 1 damage to target creature or player). If you attack with a creature and it's blocked by multiple Devil tokens, you can assign all combat damage to one to avoid getting the death triggers from the rest.

@Hackworth Okay, that exact thing happened to me (slight misplay) i blocked a 2 health creature with 2 devils and left the other one unblocked (which would die because of the death triggers), but the opponent just killed one and I was sad


text:
Does a creature's ability trigger while it's in the hand or graveyard?
When a creature card like Agent of the Fates is not on the battlefield but for example in the hand or graveyard, will its triggered abilities still trigger when I play Animate Dead or my opponent plays Ostracize and chooses the Agent?

Welcome to this site.
You ask a question, and hopflly you will get some answers.

@doppelgreener OP's asking whether triggered abilities of permanents trigger when the cards representing them are not on the battlefield, i.e. when they're not permanents, i.e. no.

No.
Unless specifically stated otherwise in the ability text, or they could only function from another zone (such as in Flashback), abilities of creatures, lands, artifacts, etc., only work while they are permanents, i.e. when they are on the battlefield.
Also note that Ostracize does not even target creatures, it only targets opponent players, so it could never trigger the Agent.

There's one exception to the "explicitly stated" rule that's tangential to the actual question but may still be worth bringing up: characteristic-defining abilities. The rules text doesn't say so, but they function everywhere, including exile/outside of the game. CR 604.3 defines this type of ability, but the most obvious one is possibly "CARDNAME's power and toughness are each equal to...". Several other abilities key off of P/T for e.g. fetching or countering, and it's worth noting that those values are set in all zones and update as a state-based action.

@matthias I'm not sure if this has changed since last year, but characteristics defined by static abilities are updated continuously, not as a state-based action.


text:
What happens if I cast disenchant in response to the triggered ability of animate dead?
My opponent has Griselbrand in his graveyard and casts Animate Dead.
It resolves, causing the triggered ability to go on the stack which would bring Griselbrand back to play. In response, I Disenchant Animate Dead.
What happens?
My friend believes it would be the same as the bouncing Oblivion Ring trick-- the triggered ability would still activate, causing Griselbrand to come out, but the aura would be gone before its "leaving the battlefield" trigger has anything to target: i.e. Griselbrand comes out and stays out, while Animate Dead goes to the graveyard.
This sounds right. Is it?

If you destroy Animate Dead before the creature card it enchants returns to the battlefield, the creature card will remain in the graveyard and never enter the battlefield.
Read the text of Animate Dead carefully:

When Animate Dead enters the battlefield, if it's on the battlefield, it loses "enchant creature card in a graveyard" and gains "enchant creature put onto the battlefield with Animate Dead."

If Animate Dead enters the battlefield, it triggers its ETB ability. If someone responds with a Disenchant to destroy it, then by the time the ETB resolves, the aura will no longer be on the battlefield and the ability does nothing on resolving.
The Oblivion Ring trick you mentioned works only because its ETB ability does not have that condition.

I see now that it means "When ~ enters the battlefield, and if it's still on the battlefield at the time this ability resolves..." but holy shit is the chosen phrasing counter-intuitive.


text:
Can you sacrifice Animate Dead AND the creature it enchants?
My opponent has a creature with Animate Dead attached to it. I attacked him with Emrakul, the Aeons Torn, forcing him to sacrifice 6 permanents to Annihilator. He wanted to sacrifice both Animate Dead and the creature it was enchanting. I said that was impossible, as sacrificing either one would cause the other to leave the battlefield. Which of us is right?

But don't worry, you're attacking with Emrakul. You're probably going to win anyway.

Yes, if someone's asked to sacrifice multiple permanents, they can sacrifice Animate Dead and its enchanted creature.
As part of enacting Emrakul's Annihilator ability, your opponent has to to make six choices. These six choices are made all at once during resolution, and at that point they're all valid choices, and then after those choices are made, the ability dumps all six choices into the graveyard simultaneously.
Just about no triggered abilities can actually activate during effect resolution, so Animate Dead can't do its thing until that ability's fully resolved. Once it is, Animate Dead has left the battlefield and its triggered ability goes onto the stack:

When Animate Dead leaves the battlefield, that creature's controller sacrifices it.

That creature's no longer around, and nothing happens.

The answer says that you must make six choices as the ability goes on the stack, but I think you make the choices when the ability resolves. Otherwise, nice answer!

You could still sacrifice both even if is asked you to sacrifice one at a time in order, because the game state isn't checked until the ability is fully resolved. It would be a different story say if Annihilator 1 triggered six individual times

@Rainbolt Thanks for pointing that out. I've asked a question about that - I don't doubt you but I'm interested in the rules behind it, plus it seems like a good question to have on-site.

@Aducci I'll add a note about that after I clarify the situation around Annihilator


text:
Tidehollow Sculler and Ruin Processor
If I play Tidehollow Sculler and remove a card from the game then I play a Ruin Processor to put that card to the graveyard, would my opponent return the card from graveyard to hand when Tidehollow Sculler dies?

No, it would not return to your opponent's hand that way.
The reason for that is that the previously exiled card is no longer in exile. It explicitly says that it moves the card from exile to opponent's hand, but the card is no longer in exile by that time, so it's trying to do something impossible.
Also, the object represented by that card stops existing when it changes zones, and a new object is created, represented by the same card. There are some exceptions that allow objects to be tracked across zones, but none of them apply here.

400.7. An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence. There are seven exceptions to this rule:


text:
Clever Impersonator copying Liliana Heretical Healer
I'm copying my opponent's non-transformed Liliana, Heretical Healer with Clever Impersonator. Then one of my other creatures dies. Does my Clever Impersonator come back as Liliana non-transformed again or does it come back as Clever Impersonator with a possible new choice? If it comes back as Liliana do I get a zombie? It says "If you do, put a 2/2..." is it saying if I do return her, or if I do return her transformed?

You will get the 2/2 Zombie token and a fresh Clever Impersonator.
The copied Liliana's ability triggers on the other creature's death, the copied Liliana gets exiled and returns to the battlefield. However, the Impersonator only copied the front side of Liliana, the Impersonator itself can not and will not be transformed:

701.25a Only permanents represented by double-faced cards can transform. (See rule 711, “Double-Faced Cards.”) If a spell or ability instructs a player to transform any permanent that isn’t represented by a double-faced card, nothing happens.

In this case, the "If you do..." part of Liliana's ability is sufficiently satisfied as long as the card is exiled due to the effect, so a 2/2 zombie will be created. (see this post)
Upon leaving the battlefield, the Impersonator also forgot all memory of its previous existence:

400.7. An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence. There are seven exceptions to this rule:

The exceptions don't apply here. You exile the Liliana, but return the Impersonator. Therefore, after the exile it enters the battlefield as a fresh Impersonator, ready to copy another permanent.

Do the impersonator-cloned Liliana put a zombie token on the battlefield when it happens ? Just trying to clarify.

@Autar No, you get the zombie token only when you return the transformed Liliana ("If you do,...").

@Hackworth I'm pretty sure the 'If you do...' refers to exiling her, so you would get the token.

http://magicjudge.tumblr.com/post/125330140254/re-the-dfc-walkers-and-copy-effects-how-does

You should probably add that to the answer, since it's part of the question.

@diego Is there a rule that explains why this is the case? From the wording on the card it seems completely ambiguous; I would have assumed that you had to resolve the entire thing to meet "if you do."

@GendoIkari CR117.12 Some spells, activated abilities, and triggered abilities read, “[Do something]. If [a player] [does or doesn’t], [effect].” or “[A player] may [do something]. If [that player] [does or doesn’t], [effect].” The action [do something] is a cost, paid when the spell or ability resolves. The “If [a player] [does or doesn’t]” clause checks whether the player chose to pay an optional cost or started to pay a mandatory cost, regardless of what events actually occurred.

@diego Thanks. But in this case, the ability is not optional, so does the "if you do" actually change anything in this case? Is there any situation in which you wouldn't get the Zombie token if the ability has triggered?

@GendoIkari This case would fall under the 'started to pay a mandatory cost' portion. You would not get a token if for example Lili's trigger was put on the stack and in response she is removed from the battlefield, since it is no longer possible to exile her.


text:
What can I sacrifice for Champion of Stray Souls' ability?
I have a Champion of Stray Souls. It says, in short, that I can trade any number of creatures (sacrifice) for any other creatures in my graveyard.  It does not say creatures in my hand or on the battlefield, so my question  is:  Can I sacrifice creatures in my deck or in my hand (or just my hand but not my deck, etc.) to trade in for creatures in the graveyard, or just creatures on the battlefield?

You can only sacrifice creatures that are on the battlefield and you control.
From the comprehensive rules:

701.14. Sacrifice
701.14a To sacrifice a permanent, its controller moves it from the battlefield directly to its owner’s graveyard. A player can’t sacrifice something that isn’t a permanent, or something that’s a permanent he or she doesn’t control. Sacrificing a permanent doesn’t destroy it, so regeneration or other effects that replace destruction can’t affect this action.

A permanent is a card on the battlefield, such as creatures, lands, artifacts, etc.. A card that is not on the battlefield can never be a creature (land, etc), only a creature card (land card, etc.).

It might be worth emphasizing that this is part of the definition of "sacrifice" and has nothing to do with Champion of Stray Souls specifically.

@DavidZ that's because there's nothing specific about the card. It lets you sacrifice creatures. This rule tells you what you can sacrifice.

@DavidZ It's even more general, not just part of the definition of sacrifice. It's a general thing, as the answer mentions in the last sentence: "a creature" (or "other creatures" as the card in question says) always means a permanent. Creature cards elsewhere have to be referred to as creature cards. For what it's worth, the comp rules quote is "109.2. If a spell or ability uses a description of an object that includes a card type or subtype, but doesn’t include the word “card,” “spell,” “source,” or “scheme,” it means a permanent of that card type or subtype on the battlefield."

@corsiKa yes, that's exactly what I'm saying. My point is that, given the way the question is worded, I could imagine a new player missing the fact that this applies to any instance of the word "sacrifice" on any Magic card.

@Jefromi well, I was thinking of how the fact that you can only sacrifice permanents is part of the definition. But yes, the point I actually think is worth emphasizing is that you cannot sacrifice a card in your hand, graveyard, or library.

@DavidZ The answer seems pretty clear that it's making a general statement, starting with that first bolded sentence. As for which rule/definition to look at... both are worthwhile. It's good to know that when you see "sacrifice" it's going to talk about a permanent, but it's also good to get it in your head that when a card says "creature" (whether sacrificing is involved or not) it means a permanent.


text:
vedalken orrery, aura of silence in reponse to opponents enchantment
If I have a Vedalken Orrery in play, and my opponent attempts to cast an enchantment, can I flash in Aura of Silence in response and force them to either pay 2 more mana or have their spell fizzle?

By the time you get a chance to respond to the enchantment it has already been cast and all cost has been payed. It will effect all subsequent spells but not the one already cast.

No, it generally doesn't work like that.
If your opponent has priority (i.e. the right to cast spells, activate abilities, and take special actions), and all other required conditions are also met, he can start casting his enchantment. In that case, you can still respond with Aura of Silence, but it will not have any effect on that particular enchantment. The Aura already has to be on the battlefield to have an effect on spells, because once a spell is on the stack, its costs can no longer change in any way:

601.4. Casting a spell that alters costs won’t affect spells and abilities that are already on the stack.

So in the classic case with no rules-lawyering going on, you are out of luck.
That being said, in a tournament setting with strict rules enforcement, you might be able to use the shortcut rules and your opponent's sloppiness in your favor.
Shortcuts are being used in almost every turn of every match of Magic ever, and that is no exaggeration. The simple act of untapping, drawing a card, and saying "Go" to end your turn is actually a large shortcut you propose to skip over all the usually required steps and phases of a turn, and usually your opponent will accept that proposal. However, he is not required to and can demand to go back to your draw step and propose a different shortcut, which you in turn have to accept and so on.
So what does this mean for your question?
Let's say your opponent starts his turn, untaps, draws a card, then immediately starts casting his enchantment. What he actually did was he silently proposed for both of you to pass priority and continue into the first main phase, and assumed you would accept. However, it is your right to demand that both of you play out all the necessary steps, and after your opponent has drawn a card and passed priority to you, you get the opportunity to cast and resolve your Aura of Silence. Your opponent has to wait until the main phase, so if he still wants to play his enchantment, he would have to pay 2 more.
You get another opportunity to do the same thing at the end of combat. At various points during that phase, and notably at the end of combat step, players can cast instant-speed spells and activate abilities. So when creatures did attack and combat is over, and your opponent wants to cast an enchantment spell, it's clear that he has skipped to the second main phase, and you can then deny that shortcut and play your own spell at the end of combat step.

@ikegami "by the time you see AND have the opportunity to react". I am answering for the normal case, when the opponent indeed has priority and has cast his spell, to which OP reacts with Aura of SIlence. In that context, yes, always.

I will edit my answer to clear up any confusion. "Seeing my opponent cast a spell" can indeed mean different things. If my opponent has priority etc. and starts casting a spell, then he really is casting a spell and there is no way to change the costs of that spell. If he cannot legally cast a spell but starts doing so anyway, like Ikegami showed with his example, then I can demand to rewind the game to a point that has not explicitly happened yet. Of course, if that point is not the earliest possible, then I also just propose a shortcut that could be further shortened, and so on.

(Earlier comment withdrawn, and I'll remove this one too.)

"In a tournament setting with strict rules enforcement" is wrong. Shortcuts are part of the core rules. All the TR does is create some default shortcuts that aren't relevant here.

@Hackworth Are you certain about "it is your right to demand that both of you play out all the necessary steps"? I was under the impression that you either had to accept the shortcut or deviate from it at a specific point. (Sorry for the barrage of comments - your edit added a lot of new content.)

@Rainbolt I am certain. That "specific point" can be any point I want, as long as that point is a place where a player has priority, i.e. the right to actually play the game. My proposed shortcut can have length 1, i.e. the next time someone has priority, i.e. what would happen anyway even without shortcuts. That way I can force everyone to explicitly decide on priority, at every single step of the game. Any other conclusion would be absurd, when you think about it - if I could not demand it, it would mean you could take away anyone's and everyone's decision about passing priority or not.

@Hackworth In order to shorten a shortcut, you must deviate from it. You cannot shorten a shortcut by doing nothing. 716.2c [...] If the shortcut was shortened from the original proposal, the player who now has priority must make a different game choice than what was originally proposed for that player.

Here's a better explanation.

Casting a spell while someone "attempts to cast an enchantment" makes no sense. You can't interrupt casting. (No player gets priority during the casting of a spell.)
If you cast Aura of Silence before they cast the enchantment, they will have to pay the extra cost.
If you cast Aura of Silence after they cast the enchantment, they won't have to pay the extra cost.

601.4. Casting a spell that alters costs won’t affect spells and abilities that are already on the stack.

So the question becomes the following: Can you cast your Aura of Silence before he starts casting his enchantment once he's made it clear he wants to cast the enchantment? Probably not, but it is possible.
What you have to determine when they start casting the enchantment is whether they'res actually casting it or if they are proposing a shortcut. If they are proposing a shortcut, you may be able to shorten it to cast your spell[1]. But if the game is currently in one of their main phases, there's probably nothing you can do. I suspect that's the case here, though you didn't provide enough information for us to know decisively.

For example, if they start casting the enchanment immediately after dealing combat damage, they are actually proposing the following shortcut:

Both players pass priority until my main phase.
Then, I cast this enchantment.

Then, you could shorten the shortcut to cast Aura of Silence in the End of Combat Step. (They wouldn't be obliged to cast the enchantment if you did that.)
If they knew you might be planning to do that, they'd ask "main phase?" and wait for you to acknowledge. Once you do, they can cast their enchantment without you being able to cast your Aura of Silence first.

Just to add to the answers already given. You have to pay the costs of spells including any additional cost or "taxes" and if you cannot then the spell does not even get to be cast.
There is no retroactive countering of spells that have not paid some sort of cost. You can cast spells only if you can pay its cost in its entirety.


text:
Liliana, Untouched by Death and Ashes of the Fallen Interaction?
How does Liliana, Untouched by Death's +1 ability interact with Ashes of the Fallen (naming Zombie)?
This question was asked about a similar interaction between Ashes and Necromancer's Stockpile, but I'm not sure if there was a definitive answer.
I'm curious if the wording of the cards makes a difference here. For example, Necromancer's Stockpile states "if the discarded card was a Zombie card" (i.e. before it was discarded) whereas Liliana's +1 states if a milled card "is a Zombie card" (i.e. after it was milled into the graveyard).

Liliana, Untouched by Deaths' +1 considers every moved creature card a Zombie card with Ashes of the Fallen naming Zombie on the battlefield.
By the time Liliana's ability checks the three cards for their type, they are already in the graveyard, and the creature cards among them are Zombies because of Ashes of the Fallen. Ashes of the Fallen has a static ability which creates a continous effect.

604.2. Static abilities create continuous effects, some of which are prevention effects or replacement effects. These effects are active as long as the permanent with the ability remains on the battlefield and has the ability, or as long as the object with the ability remains in the appropriate zone, as described in rule 113.6.

Since activated abilities resolve in the order they're written, the cards are Zombies as soon as they enter the graveyard. The next instruction checks for the cards being Zombies and finds that they are.
I can't explain the difference in wording ("was" and "is") between Liliana and Stockpile, even though they seem to do very similar things: move cards from a hidden zone to the graveyard and check if they had certain properties. Generally, there seems to be a difference between "If something was a" and "If something is a", but I don't understand the pattern enough to draw general wording rules from it.

I believe the reason for the "was" in necromancer's stockpile is because the card that's discarded is part of the cost and not part of the effect.

After looking at the cards listed for "If something was a" I think the purpose is to make sure that they check the state of the card before the effect took place. I know I read about this before for death triggers that need to check the previous state of the card but I don't remember much about it.

603.10 is an example of how triggers can look back in time https://blogs.magicjudges.org/rules/cr603/ so I imagine similar wording exist for other types of effects like activated abilities

Triggers "looking back in time" is a rule specific to triggered abilities: 603.10. There are not equivalent rules for other kinds of effects.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Stream of Thought/Creeping Chill Interaction?
What happens stack-wise when Stream of Thought mills Creeping Chill?
My understanding is that the triggered ability of Creeping Chill gets put on the stack but doesn't happen until Stream of Thought is finished resolving. So if you choose to shuffle back Creeping Chill with Stream of Thought, then would the triggered ability fizzle?
Any thoughts?

Note that you can only have Stream of Thought put Creeping Chill back into a library if you're targeting yourself with Stream of Thought's milling. If you're targeting your opponent, and they mill a Creeping Chill, then you can't, because Stream of Thought only allows you to place cards from your own graveyard into your library.

Creeping Chill's ability triggers and goes on the stack if Stream of Thought puts it into the graveyard. If you then shuffle it into your library, the ability will still resolve (it won't "fizzle") but it won't do anything because you can't exile the Creeping Chill. The Handling Triggered Abilities rules section has these rules:

603.2. Whenever a game event or game state matches a triggered ability’s trigger event, that ability automatically triggers. The ability doesn’t do anything at this point.
603.3. Once an ability has triggered, its controller puts it on the stack as an object that’s not a card the next time a player would receive priority. See rule 117, “Timing and Priority.” The ability becomes the topmost object on the stack. It has the text of the ability that created it, and no other characteristics. It remains on the stack until it’s countered, it resolves, a rule causes it to be removed from the stack, or an effect moves it elsewhere.

Then rule 118.12 says

Some spells, activated abilities, and triggered abilities read, “[Do something]. If [a player] [does, doesn’t, or can’t], [effect].” or “[A player] may [do something]. If [that player] [does, doesn’t, or can’t], [effect].” The action [do something] is a cost, paid when the spell or ability resolves. The “If [a player] [does, doesn’t, or can’t]” clause checks whether the player chose to pay an optional cost or started to pay a mandatory cost, regardless of what events actually occurred.

The bolded part matches the wording on Creeping Chill:

When Creeping Chill is put into your graveyard from your library, you may exile it. If you do, Creeping Chill deals 3 damage to each opponent and you gain 3 life.

This is a choice you make while the ability is resolving, so the ability will start to resolve whether or not Creeping Chill is still in the graveyard, but if it's not in the graveyard to be exiled, you can't pay that cost and the rest of the effect won't happen.

So you're saying that it can only be exiled if it's in the graveyard? Or that it can't be exiled from the deck? You have a leap of logic that I can't follow, can you make that explicit in your good answer, and make it great? :)

I mean, I read it like this: "when A/goes-in-graveyard", A happened, "then you may B/exile-it", can't they exile it if it goes out of the graveyard? (e.g. into play, or into the deck), "if you do, C/activate-ability", and if they do exile it it triggers.

@ANeves you can't exile it from the deck because cards in the deck are not visible.

@Taemyr More accurately, you can't exile it from the deck because the card that is in the deck is not the same object as the one that was in your graveyard; and the object that was in your graveyard is the only object that the instructions are telling you that you may exile. Even if the library were not a hidden zone that wouldn't change things.

While exiling could be considered a "cost" in the general English sense of something that must be done to achieve the desired result, it's not a cost in the MtG rules specific sense.

It is a cost in the MtG rules sense. That is exactly what the quoted rule 118.12 says.

If you self-mill Creeping Chill and immediately shuffle it back with Stream of Thoughts, you can't choose to exile Creeping Chill, and you won't get its effect.
That is because Creeping Chill is a triggered ability rather than a replacement effect on a static ability.

603.1. Triggered abilities have a trigger condition and an effect. They are written as “[When/Whenever/At] [trigger condition or event], [effect]. [Instructions (if any).]”

The ability does not fit any replacement effect wording pattern:

614.1a Effects that use the word “instead” are replacement effects. Most replacement effects use the word “instead” to indicate what events will be replaced with other events.

As you correctly note, by the time Creeping Chill's ability resolves, the object it's referring to (the object represented by the card in the graveyard) is no longer in the expected zone (graveyard), because the card was already shuffled back into the library.
As for technicalities: First, Creeping Chill's ability triggers, but does not yet go on the stack while Stream of Thought is still resolving. It goes on the stack once SoT has finished resolving and the next player would get priority.

603.2. Whenever a game event or game state matches a triggered ability’s trigger event, that ability automatically triggers. The ability doesn’t do anything at this point.
603.3. Once an ability has triggered, its controller puts it on the stack as an object that’s not a card the next time a player would receive priority. See rule 117, “Timing and Priority.” The ability becomes the topmost object on the stack. It has the text of the ability that created it, and no other characteristics. It remains on the stack until it’s countered, it resolves, a rule causes it to be removed from the stack, or an effect moves it elsewhere.

Second, Creeping Chill's ability would not "fizzle". A spell or ability fizzles if all its targets become illegal between triggering and resolving. Since Creeping Chill's ability has no targets, they can't become illegal. The ability will resolve normally, you just can't choose to exile the card.

608.2b If the spell or ability specifies targets, it checks whether the targets are still legal. A target that’s no longer in the zone it was in when it was targeted is illegal. Other changes to the game state may cause a target to no longer be legal; for example, its characteristics may have changed or an effect may have changed the text of the spell. If the source of an ability has left the zone it was in, its last known information is used during this process. If all its targets, for every instance of the word “target,” are now illegal, the spell or ability doesn’t resolve. It’s removed from the stack and, if it’s a spell, put into its owner’s graveyard. Otherwise, the spell or ability will resolve normally. [..]

As an aside, if Creeping Chill's triggered ability was worded as a static ability with a replacement effect, it could be made to work with Stream of Thoughts.


text:
Double Gleam of Authority interaction
If 2 copies of Gleam of Authority are on the board would that mean that +1/+1 counters would bounce back and fourth to infinity?

You can create links to card by typing [mtg:Card Name]

Note that this other question is the same as this one, and is already closed as a duplicate of the question I linked.

So if one creature had a 1/1 counter and a 2nd creature had gleam of authority the 2nd creature would get a counter and a third creature with GOA would also get a counter but the 2nd creature wouldn't get a counter for the third creature getting a counter? If that makes sense lol

No, Gleam of Authority doesn't add counters (except with its Bolster ability), it just grants +1/+1 without using counters, so if the Gleam of Authority is removed from the creature it loses the +1/+1, whereas if it added counters the creature would not lose them when Gleam of Authority is removed.

No, it would not.
You misunderstood how Gleam of Authority works. Gleam does not place +1/+1 counters on the creature it enchants, it simply gives it +1/+1 for each counter on other creatures. The difference is that counters remain on the permanent they are placed on while the +1/+1 granted by an enchantment disappears when the enchantment is destroyed.


text:
Why does Bonded Horncrest have to attack with another creature?
Bonded Horncrest has the ability:

Bonded Horncrest can't attack or block alone.

However, if Bonded Horncrest must attack, the ruling is:

If an effect says that Bonded Horncrest attacks or blocks if able and you control another creature able to attack or block, you must attack or block with Bonded Horncrest and that creature.

Why must you attack with both creatures if only Bonded Horncrest must attack?  Isn't attacking with another creature an optional "cost" that you don't have to pay (like having to pay {1} to attack)?

The ruling is correct, you have to attack with that creature if you can make it happen.

508.1c The active player checks each creature he or she controls to see whether it’s affected by any restrictions (effects that say a creature can’t attack, or that it can’t attack unless some condition is met). If any restrictions are being disobeyed, the declaration of attackers is illegal.

Note that an ability like "x:(this) loses defender" such as on Gargoyle Sentinel is an ability that indirectly lifts the restriction, but that ability is not part of the restriction because it doesn't follow the wording as described in 508.1c, and therefore you wouldn't have to pay the cost of that ability.

508.1d The active player checks each creature he or she controls to see whether it’s affected by any requirements (effects that say a creature must attack, or that it must attack if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of attackers is illegal. If a creature can’t attack unless a player pays a cost, that player is not required to pay that cost, even if attacking with that creature would increase the number of requirements being obeyed.

Bonded Horncrest being unable to attack or block alone is a restriction. Declaring another creature as an attacker or blocker is not a cost, as in cost:ability, but simply fulfilling the condition to meet the restriction.
Bounded Horncrest being forced to attack is a requirement.
Therefore, if you do not enable Horncrest to attack through declaring another creature, you disobey one requirement, but you could obey it by meeting the condition of the restriction. Therefore, you disobey more requirements than you had to, and therefore not attacking with Horncrest is illegal.


text:
What would happen if you target a animated land with Mirrorweave?
If I target an animated Mutavault with Mirrorweave, will all creatures become lands until end of turn?

Yes, the other creatures will be Mutavault lands until end of turn, but they will not be 2/2 creatures. You can, however, animate each of those lands through their copied animation ability.
A copy effect only copies what's printed on the original card and other copy effects, but no other effects. The animation effect from Mutavault is such an "other effect" that will not be copied. The example given in the rule covers pretty much exactly your situation.

706.2. When copying an object, the copy acquires the copiable values of the original object’s characteristics and, for an object on the stack, choices made when casting or activating it (mode, targets, the value of X, whether it was kicked, how it will affect multiple targets, and so on). The “copiable values” are the values derived from the text printed on the object (that text being name, mana cost, color indicator, card type, subtype, supertype, rules text, power, toughness, and/or loyalty), as modified by other copy effects, by its face-down status, and by “as . . . enters the battlefield” and “as . . . is turned face up” abilities that set power and toughness (and may also set additional characteristics). Other effects (including type-changing and text-changing effects), status, and counters are not copied.
Example: Chimeric Staff is an artifact that reads “{X}: Chimeric Staff becomes an X/X artifact creature until end of turn.” Clone is a creature that reads, “You may have Clone enter the battlefield as a copy of any creature on the battlefield.” After a Staff has become a 5/5 artifact creature, a Clone enters the battlefield as a copy of it. The Clone is an artifact, not a 5/5 artifact creature. (The copy has the Staff’s ability, however, and will become a creature if that ability is activated.)

Can you, as a respond to creatures attack you. Animate your land and cast mirrorweave? What happens to the attacking "lands"?

@Taacoo I believe that creatures which stop being creatures are removed from combat. I don't have a rules citation for it though.

@Taacoo Might be worth a new question ;)

Yes, and they won't be creatures until their animate ability is activated.
Note that (ex-)creatures that were summoning-sick will be able to tap for {C}, because the tap restriction applies to creatures.


text:
Does my Dryad Arbor stop being a creature with a Blood Moon in play?
Does my Dryad Arbor stop being a creature with Blood Moon in play?

It does not.
Taken from the rulings found here:
http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=282542
"If Dryad Arbor is changed into another basic land type (such as by Sea’s Claim), it continues to be a creature and a Dryad."
Blood Moon would turn it into another basic land type, in this case a Mountain, so it would continue to be a creature and a Dryad.
========
Edit:
With regards to Blood Sun, this too will not stop Dryad Arbor being a creature.
Blood sun simply removes any abilities that are not mana abilities (i.e. abilities that generate mana). Being a creature is not considered an ability, but a type, so Dryad Arbor would still be considered a creature. See below:
305.7. If an effect sets a land’s subtype to one or more of the basic land types, the land no longer has its old land type. It loses all abilities generated from its rules text, its old land types, and any copy effects affecting that land, and it gains the appropriate mana ability for each new basic land type. Note that this doesn’t remove any abilities that were granted to the land by other effects. Setting a land’s subtype doesn’t add or remove any card types (such as creature) or supertypes (such as basic, legendary, and snow) the land may have. If a land gains one or more land types in addition to its own, it keeps its land types and rules text, and it gains the new land types and mana abilities.

Is the same true when you have a blood sun in play?

@Neil You should ask that in a new question. Adding additional question scope like this in comments isn't considered good practice on SE.

For an even more recent example, Alpine Moon also would not keep Dryad Arbor from being a creature since it only causes them to "lose all land types and abilities". So Dryad Arbor would no longer be a Forest, but it would still be a land creature.

No, it will stay a creature.
Blood Moon only affects the subtype and original/copied abilities of affected permanents.
The only difference is that instead of being a

Land Creature - Forest Dryad,

it will be a

Land Creature - Mountain Dryad.

If Dryad Arbor gained any other abilities later, such as flying, it will retain them. It merely loses the "{T}: {G}" ability granted to it from the Forest basic land type, and instead gains "{T}:{R}" from the Mountain basic land type.

305.7. If an effect sets a land’s subtype to one or more of the basic land types, the land no longer has its old land type. It loses all abilities generated from its rules text, its old land types, and any copy effects affecting that land, and it gains the appropriate mana ability for each new basic land type. Note that this doesn’t remove any abilities that were granted to the land by other effects. Setting a land’s subtype doesn’t add or remove any card types (such as creature) or supertypes (such as basic, legendary, and snow) the land may have. If a land gains one or more land types in addition to its own, it keeps its land types and rules text, and it gains the new land types and mana abilities.

As an example, if you had Dimir Doppelganger copy a Dryad Arbor, the resulting creature would be a 1/1 Mountain Dryad without any abilities except the red mana ability as long as Blood Moon is on the battlefield; specifically, the dryad would lose the Doppelganger's ability to switch to another creature.

Does the same ruling apply when you have a blood sun in play?

@NeilMeyer no. Blood Sun doesn't interact with Dryad Arbor at all, since its only ability is a mana ability ({T}: Add {G}).

If you copy Dryad Arbor with the Doppelganger, it grants the ability to itself after the copying process. This is then no longer part of its text. So the "doesn’t remove any abilities that were granted to the land by other effects" exception mentioned in 305.7 applies. The Doppelganger keeps his ability to exile and thus copy other creature cards from graveyards.

@M.Herzkamp 706.9. Copy effects may include modifications or exceptions to the copying process.
706.9a Some copy effects cause the copy to gain an ability as part of the copying process. This ability becomes part of the copiable values for the copy, along with any other abilities that were copied. Gaining the Doppelganger ability is part of the copy effect and therefore erased by Blood Moon.

Folks, please. We shouldn't be letting people ask completely out-of-scope follow-up questions in the comments. That's not how we do things, it's considered a bad faith use of the network and we should not be enabling it. @NeilMeyer Please either ask a new question so we can store these answers permanently or update your question if you think it should also cover the Blood Sun scope.


text:
Does humility stop my Dryad Arbor from being able to tap for mana?
Does Humility remove the mana ability of land creatures such as Dryad Arbor?

Yes. A land creature is a creature, mana abilities are abilities, and Humility removes all abilities from all creatures.
A creature is a creature, no matter what other types it may have:

300.2. Some objects have more than one card type (for example, an artifact creature). Such objects combine the aspects of each of those card types, and are subject to spells and abilities that affect either or all of those card types.

Mana abilities are abilities:

605.1. Some activated abilities and some triggered abilities are mana abilities [..]

The inherent mana ability of lands with a basic land type, such as Dryad Arbor, can be treated as if it was printed on the card:

305.6. [..] An object with the land card type and a basic land type has the intrinsic ability “{T}: Add [mana symbol],” even if the text box doesn’t actually contain that text or the object has no text box. [..]

In the layer system that determines an object's characteristics, its base characterstics (the copiable values) are set in layer 1, animation effects happen in layer 4, and finally Humility removes all abilities from creatures in layer 6:

613.1a Layer 1: Rules and effects that modify copiable values are applied.

613.1d Layer 4: Type-changing effects are applied. These include effects that change an object’s card type, subtype, and/or supertype.

613.1f Layer 6: Ability-adding effects, keyword counters, ability-removing effects, and effects that say an object can’t have an ability are applied.

Therefore, Humility removes the mana-generating ability from land creatures, whether they are inherently creatures such as Dryad Arbor or they have been animated otherwise.


text:
How does doubling of life work when you are at negative life?
If I'm at -3 life and not dead from let's say a Platinum Angel. If I play a Beacon of Immortality, do I go to 6 life? Is it as simple as the primary school maths indicates or is there some other consideration?

You would go to negative 6 life.
When calculating or comparing numbers, the game uses the actual value of whatever is involved in the calculation. Usually the result of such a calculation can't be negative, but doubling a player's life is one of the few exceptions:

107.1b Most of the time, the Magic game uses only positive numbers and zero. You can’t choose a negative number, deal negative damage, gain negative life, and so on. However, it’s possible for a game value, such as a creature’s power, to be less than zero. If a calculation or comparison needs to use a negative value, it does so. If a calculation that would determine the result of an effect yields a negative number, zero is used instead, unless that effect doubles or sets to a specific value a player’s life total or a creature’s power and/or toughness.

Indeed, the rulings on Beacon of Immortality's gatherer page says "If you double a negative life total, you do the real math. A life total of -10 becomes -20."

The way power creep is going, I bet Wizards will eventually print a card that triples your life total. I wonder what the odds are that they forget to update 107.1b or make a ruling to handle this edge case?


text:
Nivmagus Elemental interactions with nivix cyclops, guttersnipe, and young pyromancer
if I exile a spell with Nivmagus Elemental, will the spell still cause the abilities of Nivix Cyclops, Guttersnipe and Young Pyromancer to trigger?

Yes. All of your examples trigger when you cast a spell. They happen whether that spell resolves or not.
I have seen multiple EDH decks built specifically around cards such as those and Cipher, where the intent is to exile spells with Nivmagus whenever you don't really need them.

The question doesn't make any sense because the casting of the spell and therefore the triggering of Nivix Cyclops's ability happens before you even have a chance to activate Nivmagus Elemental's ability.
Nivmagus Elemental's ability requires that the spell already exist on the stack, which means it requires that the spell must already have been created through casting or copying before it can be activated, which means Nivix Cyclops's ability would already have been triggered and would already have been placed on the stack by the time you can use Nivmagus Elemental's ability. In fact, you can even let Nivix Cyclops's ability resolve before you activate Nivmagus Elemental's.
Nivmagus Elemental's ability won't rewrite history so Nivix Cyclops never triggered, and it won't remove Nivix Cyclops's ability from the stack. As such, it won't stop Nivix Cyclops's ability from resolving.

"ability happens you have a chance" - I think you missed a word!

@Jonathan Hobbs, Fixed.

Yes, because you cannot exile the spell with Nivmagus Elemental until the triggered abilities have already been placed on the stack (and if you want, resolved).
From the comprehensive rules:
As soon as you cast the spell (pay costs, select targets, place the spell on the stack) the triggered abilities are placed on the stack.

601.2h Once the steps described in 601.2a-g are completed, the spell becomes cast. Any abilities that trigger when a spell is cast or put
  onto the stack trigger at this time. If the spell's controller had
  priority before casting it, he or she gets priority.

Steps 601.2a-g have been omitted and involve selecting legal targets and paying costs for the spell.
After the spell and triggered abilities are on the stack & you get priority back (either by already having it, or having your opponent pass it to you). You can then activate Nivmagus Elemental's ability. Because the triggered abilities are already on the stack they will resolve even though the spell that triggered them does not resolve.


text:
Kozilek vs Stonecloaker
So what i wanted to ask is this... Kozilek, Butcher of Truth goes to graveyard and I have Stonecloaker in hand!
My queston is can I exile Kozilek with my Stonecloaker before Kozileks ability resolves ?

Yes you can. Because when Kozilek goes to the graveyard, the "shuffle it into library" goes on the stack. Before that resolves, you can exile it from the graveyard with Stonecloaker.
The "shuffle it into library" is a so called triggered ability, therefore it goes on the stack and you can put your (instant or flash) spells on the stack before resolving that ability.

But wouldn't the stack resolve anyway? So you exile Kozilek, but then afterwards the "shuffle into library" still resolves since it is on the stack? Then all you have done is made sure Kozilek doesn't get placed in your library, but that is all? Wouldn't it be alot easyer to use the return a creature you control before damage is dealt and Kozilek dies? You'll still have him in your hand then.

@Lyrion, Yes, the "shuffle your graveyard into your library" ability will still resolve. I'm not sure what you mean by your "wouldn't it be a lot easier" comment. I assume the OP is trying to exile his opponent's Kozilek, not his own, in which case Kozilek isn't a valid choice for the "return a creature you control" ability.

@Kevin That is true did not think about that.

yea I was thinking about opponents Kozilek, we have a EDH group where Kozileks shuffle ability gives our friend infinite recursion, so I was wondering on how to remove him :) Thank you guy


text:
Will Enduring Ideal and Dovescape played together leave Enduring Ideal countered?
I play Enduring Ideal and search and play Dovescape.
On the next upkeep Enduring Ideal should be cast again.
Is Enduring Ideal countered by Dovescape, or can I search for other enchantments?

No, a copy of Enduring ideal won't be countered by Dovescape because the copy isn't cast.
The reminder text for Epic says:

At the beginning of each of your upkeeps, copy this spell except for its epic ability.

However, copies are not cast and thus are not affected by Dovescape:

706.10. To copy a spell, activated ability, or triggered ability means to put a copy of it onto the stack; a copy of a spell isn’t cast and a copy of an activated ability isn’t activated. [...]

Thanks for editing and answer! I found the card link is not working (Not showing any card) What tag or code to use for linking card easily for next time?

Don't know, the links are working for me. The format is [mtg:cardname], though certain special characters like ' cause problems.

Enduring Ideal is not cast, since you cannot cast spells, a copy of it (without Epic) is just put on the stack. Since it is not being cast Dovescape doesn't care about it and it is not countered.


text:
List of blue counterspells that can be played for free?
I looked around on the internet for a list of blue counterspells in Magic: the Gathering that can be played for no mana. (Alternative costs or a mana cost later is fine.) Does anyone have a list or know where to find a list?

No mana ever, or just no mana right now? That would affect whether things like Pact of Negation appear on the list.

Ones that can be played for no mana right now.

Does it have to be a spell?  counterbalance is an enchantment that costs zero mana (once it's on the battlefield) to counter spells.  There are several other permanents that can be used to counter spells.

There are 8 Blue counterspells that can be cast for alternate costs:

Admiral's Order (The alternate cost still costs {U})
Daze
Disrupting Shoal
Fierce Guardianship
Foil
Force of Negation
Force of Will
Thwart

I found these with the following search on Scryfall:

(o:"rather than pay ~'s mana cost" or o:"you may cast this spell without paying its mana cost") o:counter c:u

This searches for spells with alternative costs (first clause) with "counter" in the text (second clause) that are blue (third clause).
In addition, Mental Misstep only costs {UP}, so you can pay life instead of mana.
If you also want to count spells with mana that needs to be paid later, Pact of Negation has a mana cost of {0}, but it also says

At the beginning of your next upkeep, pay {3}{U}{U}. If you don't, you lose the game.

Besides that, there are 3 spells that can be used to effectively counter a spell that can be cast without paying any mana:

Commandeer lets you take control of a spell and change its targets. You can choose a target that is (more) beneficial to you, or, if the other spell is a counterspell, you can redirect it to target Commandeer so that it is countered for having an illegal target after Commandeer leaves the stack.
Mindbreak Trap lets you exile any number of spells, but you can only cast it for free if your opponent has cast at least 3 spells this turn.
Misdirection also lets you choose new targets for a spell. Of course, it is only useful for dealing with targeted spells, and you can use it to deal with counterspells the same way you can use Commandeer.

Fierce Guardianship can be added to the list if you're playing Commander.

I added that. I missed it because it uses a different alternate cost wording.

Yeah, wasn't sure where you wanted to add it because your description below the list is now slightly incorrect. Also maybe Pact of Negation as mentioned in comments? (Oh, you updated that as well!)

Depending on how you read it, the description is still right. The whole parenthesized expression searches for spells with alternative costs, it just now includes two different wordings for them. Pact of Negation was there the whole time. The only change I made today was to expand the list at the beginning.


text:
Damage dealt by creatures and the order of damage
So I had an interesting one recently at our local win a box tournament.
Scene - I have 12 life remaining, my opponent has 9. I have an un-monstrous Arbor Colossus and a Polukranos, World Eater(monstrous 4), my opponent has no board at all apart from lands.
It's my turn, I attack with both my creatures. My opponent casts Titan Strength on my Polukranos and then casts Deflecting Palm on Polukranos so I am now dead to my own Polukranos (12/12).
The game ended here because my opponent started shouting so loud about how he had won with such a play that I missed a vital part of the board.
I could have used the monstrous ability on my Arbor Collosus and made that a 9/9. So I have this question:-
If I had used my monstrous on the Arbor Collosus - would that make the game a draw (we both die at the same time) or could I have had my creatures deal their damage in an order of my choosing, and thus win the game before being killed by my own Polukranos??

All combat damage is dealt at the same time, so yes, it would have been a draw.
Deflecting Palm is a replacement effect (instead of your opponent, you take damage the moment combat damage is dealt). This happens at the same time for all combat damage. So, the Polukranos damage hits you the same time the Arbor Colossus damage would have hit your opponent.
/edit: last paragraph was incorrect

And (for the last sentence of his question) it wouldn't have mattered if he could decide the order in which damage was dealt, because the players only actually lose the game the next time state based actions are checked. Right?

This was game 2, the final game ended in a draw for a draw overall. I had won the first game.

Regarding your final point: matches are played until time runs out or a player has won 2 games. If there is still time left and the current game score is 1 win for either or both players and any number of draws, the players should begin another game.

This is cool to know.  Do you have a reference for the "If damage is prevented this way, then do X" ability being a replacement effect and not a triggered ability?  I would have guessed it was triggered, thus OP would have won while it was still on the stack, but I'm a bit rusty at this...

@DanStaley - you can tell by the wording: "When/Whenever/At [trigger], [event]" are triggered abilities. Replacement effects are commonly phrased using "If [something], [something else] instead", and "~ enters the battlefield with/as ..." though there are several other wordings, too. See rule 614 for the full list.

Can you quote a source for the match score paragraph? I thought that a draw would be recorded as 0-0-1, so if the previous score for the match was 1-0-0, now the score would be 1-0-1 and the match would continue.


text:
Jace memory adept 0 ability with less than 10 cards
If I activate Jace, Memory Adept's 0 ability (Target player puts the top ten cards of his or her library into his or her graveyard.). What happens if the target player has less than 10 cards in his library? Does he lose the game? Or does he lose the game on next draw step? Or the ability doesn't work?

The player has to put as many cards as possible, up to 10, into his or her graveyard. He will lose the game whenever he tries to draw from an empty library

609.3. If an effect attempts to do something impossible, it does only as much as possible.

An empty library is not a loss condition; only drawing a card from one is. Jace's 0 ability does not make the player draw cards, it only puts them into the graveyard.

704.5b If a player attempted to draw a card from a library with no cards in it since the last time state-based actions were checked, he or she loses the game.


text:
Does ETB trigger after legendary rule resolution?
My friend is building a commander deck and wants to confirm if his latest combo is correct.
Currently, there is a Reflections of Littjara on the battlefield, and the dragon creature type has been named. If I cast Miirym, Sentinel Wyrm, the following events occur:

The Reflections of Littjara trigger, creating a legendary copy of Miirym on the stack.
The legendary copy of Miirym enters the battlefield.
The real Miirym enters the battlefield. I must sacrifice one Miirym since both are legendary.

My question is: Does the Miirym token still trigger from the enter-the-battlefield ability of the real Miirym? If it does, does this mean a non-legendary token of Miirym is created and enters the battlefield?

First thing to note: a permanent being moved to the graveyard via the legend rule is not sacrificing it. Probably doesn't matter here but has important interactions with lots of other cards.

Yes, the ability triggers and you create a non-legendary copy of Miirym.
Focusing just on the resolution of the original Miirym spell, the sequence of events goes like this:

The original Miirym spell resolves. Miirym enters the battlefield, and the ability on the existing token copy of Miirym triggers.
State-based actions are checked, and you must choose to put one of the two Miiryms into your graveyard.
The triggered ability is put on the stack.
After all players pass priority, the triggered ability resolves, creating a non-legendary copy of Miirym.


text:
Chains of Mephistopheles versus ancestral recall
There are four Chains of Mephistopheles in play. I have 5 cards in hand. I cast Ancestral Recall targeting myself.
What happens?

The Chains of Mephistopheles' replacement effects chain (heh), eventually causing you to discard your hand and mill two cards.
Chains of Mephistopheles creates a replacement effect, which replaces one event (you draw a card other than the first of your draw step) with another (discard a card, then draw a card). Since the "draw a card" part of the new event can be replaced by other Chains, they do so. Each Chains effect, in order of your choosing, happens once per extra card drawn, but no more than once - they don't chain indefinitely.
Relevant rules:

614.11a If an effect replaces a draw within a sequence of card draws, all actions required by the replacement are completed, if possible, before resuming the sequence.

That means each card draw of Ancestral Recall is handled in sequence. You don't draw 3 then start discarding/milling, you immediately start discarding for each card drawn.

614.5. A replacement effect doesn’t invoke itself repeatedly; it gets only one opportunity to affect an event or any modified events that may replace that event.

The original events are the Ancestral Recall card draws. Starting from there, you apply all the Chains, but only once each per card draw.

616.1. If two or more replacement and/or prevention effects are attempting to modify the way an event affects an object or player, the affected object’s controller (or its owner if it has no controller) or the affected player chooses one to apply, following the steps listed below. [..]

616.2. A replacement or prevention effect can become applicable to an event as the result of another replacement or prevention effect that modifies the event.

That means the replacement effects chain together, since the new event created by one Chains can be replaced by another Chains, as long as you had a card to discard and thus the first Chains caused a new card draw.
The sequence of events is the following:
You start out with 5 hand cards. On the first Recall draw, the 4 Chains will cause you to discard 4 of those cards and then draw one.
You have 2 hand cards remaining. The second draw causes 2 Chains replacements. You discard those 2 hand cards and, since you have no more cards after the 2nd Chains, you mill 1 card for the third Chains. The fourth Chains does nothing because you didn't draw a card as a result of the third.
You have no hand cards remaining. The third card drawn will cause you to instead mill 1 card from a Chains and the remaining 3 Chains do nothing.
Overall, the result is that you draw 1 card, discard your hand, and mill another 2 cards.

thank you ! that’s exactly what I thought.

@TonyS ah, this answer is actually incorrect.  I'm going to write a correction.

I would have thought that you would discard four cards then draw four cards three times.

@nick012000 That would happen if Chains' ability was triggered, but it's a replacement effect.


text:
how do Enters the Battlefield abilities resolve?
If there are two "Enters the Battlefield" triggers, what decides how they resolve?
Particularly, if I had both a Tolsimir, Friend to Wolves, and a Cathars' Crusade. If I put a wolf on the battlefield, does it fight something first, or does it gain the +1/+1 first?

Since you control both triggered abilities, you decide the order in which they go on the stack and thus the order in which they resolve.
A certain event can trigger multiple abilities, or the same ability multiple times. In that case, each player gets to order the abilities they control in the way they want. So it's your choice whether to have the wolf get the +1/+1 counter first, or have it fight first.
If multiple players put their abilities on the stack this way, the last player in turn order would resolve their abilities first, and the active player last.

603.3b If multiple abilities have triggered since the last time a player received priority, the abilities are placed on the stack in a two-part process. First, each player, in APNAP order, puts each triggered ability they control with a trigger condition that isn’t another ability triggering on the stack in any order they choose. (See rule 101.4.) Second, each player, in APNAP order, puts all remaining triggered abilities they control on the stack in any order they choose. Then the game once again checks for and performs state-based actions until none are performed, then abilities that triggered during this process go on the stack. This process repeats until no new state-based actions are performed and no abilities trigger. Then the appropriate player gets priority.


text:
Can I trigger an end of turn effect with another end of turn effect?
Say I have a Crested Sunmare, Essence Warden, and Angelic Accord out on the field and I have gained 3 life this turn. Crested Sunmare triggers, creating a 5/5 horse token. This triggers Essence Warden and gives me 1 life. Can I now create an angel token? Is the new life gain from Essence Warden still on the "current turn" or will it be on the next turn (ie. I will be able to create another horse on the next turn's end step)?

In general, all of these end step abilities trigger at the same time, and may be able to influence each other. In this specific scenario, gaining life during your end step will not cause Angelic Accord to create angel tokens.
Abilities that trigger "at the beginning of your/each end step" all trigger at the same time, and you can choose to put them on the stack in any order. However, Angelic Accord's triggered ability has what is called an "intervening if" clause, which means that if the condition isn't met, it doesn't even go on the stack in the first place (Crested Sunmare's ability works like that too, but that's not immediately relevant). So by the time Crested Sunmare's ability and then Essence Warden's abilities trigger and resolve, Angelic Accord's ability already failed to trigger, so it's too late to try to meet its condition.

Is the new life gain from Essence Warden still on the "current turn" or will it be on the next turn (ie. I will be able to create another horse on the next turn's end step)?

If an ability triggers from some event happening, the triggered ability resolves in the same step as the event that triggered it (the untap step is an exception, but that's not relevant here). In this situation, the Crested Sunmare's ability triggers and resolves in the end step, so the Essence Warden's ability also triggers and resolves in that same end step. At that point it's still the same turn as when that step started, and not the next turn.

If by the start of your end step you've only gained 3 life, then Angelic Accord's ability will not trigger. From the rulings on gatherer:

If you haven't gained 4 or more life during the turn when the end step
begins, the ability won't trigger at all. Gaining life during the end
step won't cause the ability to trigger.


text:
narset, parter of veils interaction with alms collector
If I have on the battlefield both Narset, Parter of Veils and Alms Collector, and my opponent have a trigger to draw 2, do alms collector make me draw 1?
and then narset will let the opponent draw one or not depending on if it was the first card of the turn for the opponent or not.
First answer of most people in my LGS is no, because of rules 101.2 and 614.17c. and it make sens
But I think yes because of  121.1a and 121.1b, witch said that replacement effect that refer to the number of card are considered before individual draw (because of 616.1g), and that effect that restrict the number of draw applies to individual draw.
But I'm not 100% certain because 614.q7 say "can't" effect are not replacement effect
I am missing something?

You win for posting a question which two of our top three users on the site disagree on! (Edit: one of those answers has now been deleted!)

You get to draw a card from Alms Collector's ability in any case.
You are correct that Narset's passive ability is not a replacement effect, because of 614.17, but that has no relevance here - 121.2a solely exists to enable the Alms Collector replacement effect, and its second clause is not limited to replacement effects.

121.2a An instruction to draw multiple cards can be modified by replacement effects that refer to the number of cards drawn. This modification occurs before considering any of the individual card draws. See rule 616.1g.

So what happens is that Alms Collector replaces the multiple card draw event before Narset is even considered because of the second clause in 121.2a. You and your opponent each draw 1 card, and 121.2b is checked now because of Narset.

121.2b Some effects say that a player can’t draw more than one card each turn. Such an effect applies to individual card draws. Instructions to draw multiple cards may still be partially carried out. However, if an effect offers the player a choice to draw multiple cards, the affected player can’t choose to do so. Similarly, the player can’t pay a cost that includes drawing multiple cards.

We are already down to individual card draws because of Alms Collector, so only the first 2 clauses are relevant. If your opponent already drew their card for the turn, Narset will prevent their Alms Collector card draw.
In any case, you will get to draw your Alms Collector card:

609.3. If an effect attempts to do something impossible, it does only as much as possible.


text:
When Kardur, Doomscourge is active, do myriad-empowered creatures create tokens and deal damage to the Kardur player upon attacking?
In an EDH game, when an opponent controls Kardur, Doomscourge with its enters-the-battlefield (ETB) ability activated, if I attack another opponent with my creatures that have myriad (courtesy of Legion Loyalty), does myriad generate attacking tokens for the Kardur player, and does this also result in dealing damage to them?

Myriad doesn't care about Kardur's ability. Myriad tokens are created for every opponent even if they have Kardur's ability active.
Kardur's ability creates requirements that force your creatures to attack, and attack another player, if able. However, if an effect (such as Myriad) creates tokens that are already attacking, those creature tokens skipped the "declare attackers" part of combat and are thus unaffected by the requirement to attack another player. Same goes for effects that put a creature card onto the battlefield tapped and attacking, such as Winota, Joiner of Forces.

508.4c A creature that’s put onto the battlefield attacking or that is stated to be attacking isn’t affected by requirements or restrictions that apply to the declaration of attackers.

Once a creature is attacking by whatever means, it will participate in combat and deal damage as usual.

Ninjutsu is a bad example, since the ninja attacks the same player/walker that the creature it replaced was attacking per rule 702.49c: A ninjutsu ability may be activated only while a creature on the battlefield is unblocked (see rule 509.1h). The creature with ninjutsu is put onto the battlefield unblocked. It will be attacking the same player, planeswalker, or battle as the creature that was returned to its owner’s hand.

If they didn't skip being declared as attackers, there'd be an infinite loop, as the copies would have myriad and would also spawn copies.

Doesn't Kardur's ability create two requirements, not restrictions? If a creature can't attack another player (in a two-player game, or other players have Propaganda effects) it must attack Kardur's controller

@MatthewJensen thanks, fixed.


text:
Copying Éomer, Marshal of Rohan trigger on the stack
I somehow managed to trigger Éomer, Marshal of Rohan's ability and copied it 10x on the stack. Do I get 10 combat phases? Does the triggers resolve one after another or does it empty the stack when the next combat phase begins?

Yes, you get as many extra attack phases as you had copies of Eomer's ability. However, most creatures won't be able to attack more than twice, so the benefit is likely limited.
Eomer's ability doesn't trigger more than once per turn, but that does not prevent copying it. If you get a total of 10 instances of the ability, you will indeed get 10 extra combat phases.

500.8. Some effects can add phases to a turn. They do this by adding the phases directly after the specified phase. If multiple extra phases are created after the same phase, the most recently created phase will occur first.

However, you will not get to untap your creatures before each of those combat phases. All copies of the ability resolve in the same step, untapping your creatures over and over before the turn advances. The game can only advance to the next step/phase when the stack is empty and all players pass in succession.

117.4. If all players pass in succession (that is, if all players pass without taking any actions in between passing), the spell or ability on top of the stack resolves or, if the stack is empty, the phase or step ends.

Only creatures with vigilance or creatures you manage to untap another way will be able to attack more than twice during that turn. The only potential benefit of those untaps is to tap them to pay for spells and activated abilities between the resolutions.

Another benefit to having 10 additional combat steps is all the wonderful triggers you get from Helm of the Host and other friends


text:
MTG interaction between Flickerwisp and Thassa, Deep-Dwelling
Flickerwisp reads

When Flickerwisp enters, exile another target permanent. Return that card to the battlefield under its owner's control at the beginning of the next end step.

Thassa, Deep-Dwelling reads

At the beginning of your end step, exile up to one other target creature you control, then return that card to the battlefield under your control.

Can I exile another creature with Flickerwisp's ETB effect (e.g. Ravenous Chupacabra), and have the Chupacabra come back at the beginning of the end step for Thassa to blink it again? More generally, this is for any blink "until next endstep" and "at beginning of end step" triggers, e.g. Eerie Interlude and Soulherder as well. Asking for my Esper blink deck.

Minor note: the text actually printed on a card is essentially irrelevant; all that matters is the Gatherer text which does state "at the beginning of the next end step" so your Flickerwisp assumption is correct.

I've taken the liberty and updated the card text to the Oracle version. As Phillip Kendall said, the printed text is not relevant.

@Hackworth Without input from OP, you may have edited away one of the main points of confusion. The printed wording could confuse a new player into thinking that "the beginning of the end step" happens before "the end of the turn". With your edit, it's now crystal clear that both triggers occur at the beginning of the end step. This distinction belongs in your answer, not in an edit to the question. Recommend rolling it back.

@Rainbolt The original question said I'm assuming this means beginning of end step, from the errata., so I didn't see confusion about the wording being the core of the question.

No, you can't blink the same creature twice in the same turn with that combo.
Flickerwisp returns the creature it exiled as a delayed triggered ability, specifically when the next end step starts. The beginning of an end step is a singular event, and by that time, the Chupacabra has not already returned; its return goes on the stack and has to resolve first. Thassa's ability triggers at the same time, but since Thassa has to target one of your creatures to even go on the stack, that creature has to exist on the battlefield already to be a legal target. Chupacabra cannot have returned by that point, so you could only target Flickerwisp or some other creature you're controlling.
The best you can do is to arrange Thassa's and Flickerwisps's abilities such that the Chupacabra returns, then Thassa blinks Flickerwisp, and the returning Flickerwisp exiles Chupacabra again so that Chupacabra returns during your opponent's end step.