text:
In what order are replacement effects chosen with respect to damage prevention?
How are replacement effects handled in multiplayer? Here's a concrete example.

Player A, B, and C are seated around the table. It is Player A's turn.
Player A has a Furnace Of Rath (double all damage), and Hostility (prevent damage to opponents, and create 3/1 tokens instead) in play.
Player A casts Volcanic Fallout (2 damage to each creature and player).
Player B responds with Hallow (prevent all damage from spell, gain life equal to damage prevented).
No one has any more responses, no other permanents are in play.

What possible different life totals and 3/1 tokens are generated when the stack resolves?
Who gets to make the choice(s)?
Do the possibilities change if Player B or Player C are the active player? What are
the possible results?

One of the rulings for Furnace of Rath states:

If multiple effects modify how damage will be dealt, the player who would be dealt damage or the controller of the creature that would be dealt damage chooses the order to apply the effects. For example, Mending Hands says, "Prevent the next 4 damage that would be dealt to target creature or player this turn." Suppose a spell would deal 5 damage to a player who has cast Mending Hands targeting himself or herself. That player can either (a) prevent 4 damage first and then let Furnace of Rath double the remaining 1 damage, taking 2 damage, or (b) double the damage to 10 and then prevent 4 damage, taking 6 damage.

This ruling is the result of Comprehensive rule 616.1

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. If two or more players have to make these choices at the same time, choices are made in APNAP order (see rule 101.4).

There are some caveats under rules 616.1a-e which specify when some effects must be applied first, but I don't see that they apply in this particular case.
So therefore, since each player and their creatures are taking damage, they each get to decide the order to apply the replacement effects. They decide how the damage will be applied to them and their creatures in APNAP order.

101.4. If multiple players would make choices and/or take actions at the same time, the active player (the player whose turn it is) makes any choices required, then the next player in turn order (usually the player seated to the active player's left) makes any choices required, followed by the remaining nonactive players in turn order. Then the actions happen simultaneously. This rule is often referred to as the "Active Player, Nonactive Player (APNAP) order" rule.

There are three replacement effects. I will summarize:

Furnace of Wrath: Double the damage.
Hostility: Prevent damage to opponent, get 3/1 token for each 1 damage.
Hallow: Prevent all damage, gain life

This means there are six possible orders the replacement effects are applied. If Hallow is applied first, the order of the next two effects is irrelevant. Each player gets to choose the order the effects are applied, and they are applied simultaneously! So it doesn't really matter who's turn it is, although that might affect each player's choices.
Volcanic Fallout does 2 damage to each creature and each player. I will use Y as the number of creatures the selecting player controls. Each player chooses the order they are applied to themselves and their creatures, so here's the possible results for replacement effects:

123: 4 tokens if player B or C, 4Y life (4 + 4Y life if player A)
132: 0 tokens, 4 + 4Y life
213: 2 tokens if player B or C, 4Y life (4 + 4Y life if player A)
231: 2 tokens if player B or C, 2Y life (2 + 2Y life if player A)
312 or 321: 0 tokens, 2 + 2Y life

Player A, presumably, wants to maximize the number of tokens generated so will choose order 123. Player B will presumably want to minimize the number of tokens and maximize their life gain so will choose 132. Player C, presumably, doesn't want tokens and wants to minimize player B's life gain so will choose 312 or 321.
But if damage is applied simultaneously, we have contradictory results! What happens? The key is CR 616.1. Player A has no control over the order of replacement effects for the damage taken by his opponents, so unless, for some reason, Players B or C want him to get tokens, he won't. Even if Player A chooses order 123, that order only applies to the damage applied to Player A and the creatures controlled by Player A.
Therefore Player A should choose order 231, 312 or 321.
Basically it's as if Volcanic Fallout was split into three spells, and each player chooses the order that the damage applied to them and their creatures is prevented and doubled. The amount of life gained by player B will be determined by the number of creatures on the board, as this will affect everyone's decision.
Whew! I hope I interpreted this right! A real head-scratcher.

Yes.  You got it right.  I read "controller" and jumped the gun.  CURSE YOU COMP RULES!!!!

I do think you can clean up the player/choices breakdown.  It is kind of hard to extract the info.  There is so much happening here.

@OrigamiRobot I'm having second thoughts. The replacement effects are applied when the damage occurs. The chooses one to apply part of 616.1 means that something that affects all damage will make further choices irrelevant. I think we both got it wrong.

Yes, but that just means one at a time.  616.1e says to repeat the procedure of choosing one.  Come to chat and we can discuss.

@OrigamiRobot right, but my point is that the procedure is repeated after the effect is applied.

As the ruling on Furnace of Wrath shows, multiple effects can apply.  616.1a-e are the process for carrying our 616.1

I like the simplicity of your answer (i haven't corrected the error in mine), but I think you made an error with respect to Hallow. Since Hallow prevents all the damage from a spell, if Player A (PA) is AP, then PB gains +8/+12 life. If PB is AP, then PB can gain +4/+6/+8/+10 life depending upon player's choosing to use Hallow to prevent damage. If PC is AP, then PB can gain +6/+10, once again depending upon player's choices. I am not sure your calculation takes that into account. PA can also end up with 0/2/4/6/8 tokens.

I love asking this question, because even people who consider themselves rules gurus usually get it wrong (including me). Final Edit: I think I have it right now.
ghoppe correctly identified the rules that handle multiple prevention effects trying to modify the same event.
Comprehensive Rules

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. If two or more players have to make these choices at the same time, choices are made in APNAP order (see rule 101.4).
101.4. If multiple players would make choices and/or take actions at the same time, the active player (the player whose turn it is) makes any choices required, then the next player in turn order (usually the player seated to the active player's left) makes any choices required, followed by the remaining nonactive players in turn order. Then the actions happen simultaneously. This rule is often referred to as the "Active Player, Nonactive Player (APNAP) order" rule.

Rule 101.4 is just reminding us what APNAP order is, and the fact that even though the choices each player make have an order, the result of their choices are applied simultaneously.
Lets Look at the actual text of each card (ignoring parts that don't apply), so that we know what the event is, and what is trying to replace that event.
(Event) Volcanic Fallout: ~this~ deals 2 damage to each creature and each player.

Furnace of Rath: If a source would deal damage to a creature or player, it deals double that damage to that creature or player instead.
Hostility: If a spell you control would deal damage to an opponent, prevent that damage. Put a 3/1 red Elemental Shaman creature token with haste onto the battlefield for each 1 damage prevented this way.
Hallow: Prevent all damage target spell would deal this turn. You gain life equal to the damage prevented this way.

The event that Volcanic Fallout is creating, can be thought of as 4 individual events that happen simultaneously. You will see later why it is easier to think of the event in these terms.

deal 2 damage to Player A
deal 2 damage to Hostility
deal 2 damage to Player B
deal 2 damage to Player C

Player A (Active Player)
Rule 101.4 lets Player A make choices first, because they are the active player. According to Rule 616.1, they may only get to make choices for replacement effects for which they are the affected player/affected object's controller. This means that they completely ignore Hostility's replacement effect, because it is only replacing damage to opponents. This leaves Player A with two choices.

13: Apply Furnace of Rath, then Hallow. (Result Player B +12 Life)
31: Apply Hallow, then Furnace of Rath. (Result Player B +8 Life)

(1)(3) If Player A chooses to apply Furnace of Rath first, the event looks like this:

deal (double) 2 damage to Player A
deal (double) 2 damage to Hostility
deal 2 damage to Player B
deal 2 damage to Player C

Then because there are still replacement effects to be applied, Player A applies Hallow to the event:

(Prevent all damage) 4 damage to Player A, Player B gains 4 life.
(Prevent all damage) 4 damage to Hostility, Player B gains 4 life.
(Prevent all damage) 2 damage to Player B, Player B gains 2 life.
(Prevent all damage) 2 damage to Player C, Player B gains 2 life.

Since Hallow prevented all damage from the event, the other players do not have a choice of applying Hostility's prevention effect.
Result Player B gains 12 Life

(3)(1) If Player A chooses to apply Hallow first, (then Furnace of Rath has no effect), the event looks like this:

(Prevent all damage) 2 damage to Player A, Player B gains 2 life.
(Prevent all damage) 2 damage to Hostility, Player B gains 2 life.
(Prevent all damage) 2 damage to Player B, Player B gains 2 life.
(Prevent all damage) 2 damage to Player C, Player B gains 2 life.

Result Player B +8 Life

Player C (Active Player)
If instead, Player C is the active player, they can choose when to apply all prevention effects, including Hostility since it is preventing damage to them. Rule 101.4 lets Player C make choices first. This leaves Player C with 6 choices for replacement effect order. (Parenthesis indicate that the replacement effect has nothing to replace for that player).
 - 123: Apply Furnace of Rath, Hostility, (Hallow). (Result Player A 4 tokens, plus Player A's choices)
 - 132: Apply Furnace of Rath, Hallow, (Hostility). (Result Player B +10 Life)
 - 213: Apply Hostility, (Furnace of Rath), (Hallow). (Result Player A 2 tokens, plus Player A's choices)
 - 231: Apply Hostility, (Hallow), (Furnace of Rath). (Result Player A 2 tokens, plus Player A's choices)
 - 312: Apply Hallow, (Furnace of Rath), (Hostility). (Result Player B +8 Life)
 - 321: Apply Hallow, (Hostility), (Furnace of Rath). (Result Player B +8 Life)

(1) If Player C chooses to apply Furnace of Rath first, the event looks like this:
 - deal 2 damage to Player A
 - deal 2 damage to Hostility
 - deal 2 damage to Player B
 - deal (double) 2 damage to Player C
(2)(3) Then Player C has two choices. Apply Hostility first, Hallow has nothing left to prevent. Proceed to Active Player A for his choices:
(2) Hostility
 - deal 2 damage to Player A
 - deal 2 damage to Hostility
 - deal 2 damage to Player B
 - (put) 4 (3/1 tokens) into play under Player A's control.
Result Player A 4 tokens, plus Player A's choices
(3)(2) or Player C Applies Hallow preventing all the spells damage (no more decisions for any other player):
(3) Hallow
 - (Prevent all damage) 2 damage to Player A, Player B gains 2 life.
 - (Prevent all damage) 2 damage to Hostility, Player B gains 2 life.
 - (Prevent all damage) 4 damage to Player B, Player B gains 4 life.
 - (Prevent all damage) 2 damage to Player C, Player B gains 2 life.
Result Player B +10 Life

(2) If Player C chooses to apply Hostility first, the event looks like this:
 - deal 2 damage to Player A
 - deal 2 damage to Hostility
 - deal 2 damage to Player B
 - (put) 2 (3/1 tokens) into play under Player A's control.
Since Player A is no longer an affected player (Hostility has prevented all damage to them and any other permanents they are the owner of).
Result Player A 2 tokens, plus *Player A's choices*

(3) If Player C chooses to apply Hallow first, the event looks like this:
 - (Prevent all damage) 2 damage to Player A, Player B gains 2 life.
 - (Prevent all damage) 2 damage to Hostility, Player B gains 2 life.
 - (Prevent all damage) 2 damage to Player B, Player B gains 2 life.
 - (Prevent all damage) 2 damage to Player C, Player B gains 2 life.
Since Hallow has prevented all damage, the remaining events have no effect.
Result Player B +8 Life

Player B (Active Player)
If Player B is the active player, they will have the exact same 6 choices for the order of replacement effects as Player C, and the same possible outcomes when Player C was the active player. The only difference is, if they choose to apply the effects of Hostility before Hallow, then they need to add in the additional choices that Player C could make (which might also lead to choices that Player A could make). This is because applying Hostility prevents all remaining damage to the affected player. If instead each player controlled some creatures, they would need to apply the effects of all three replacement effects, because Hostility only prevents damage to players leaving Hallow with the ability to prevent damage to affected creatures that the active player controls.

Conclusion
Even with such a simple example, you can tell how the order of replacement effects can have a great effect on the results of an event. With just one event, and 3 replacement effects, you have a wide diversity of outcomes. You can think of the possible outcomes as a decision tree, with the Active Player making choices and either leading to a lower branch or ending. Player A will receive either (0/2/4/6/8 tokens, and Player B will gain 4/6/8/10/12 life) all dependent upon who the Active Player is, and the decisions they make.
-Player B Active (Player A +8/+10 life or Player A +2/+4 tokens and proceed to C Active Player add results from their decisions)
-Player C Active (Player A +8/+10 life or Player A +2/+4 tokens and proceed to A Active Player add results from their decisions)
-Player A Active (Player B +8/+12 life if A is Active Player)
1) (+6/+12 from A's decisions if C is Active Player)
2) (+4/+8 from A's decisions if B is Active Player)

During all this, I forgot Hostility was a creature too (and that Player A only has two effects to choose) @ghoppe and I were talking earlier.  Would Player A make the choices for himself separate from his choices for his creature?

Also, each player always gets to make choices regardless of who the AP is.  Otherwise it would simply say the AP chooses the order.

I disagree.  Choosing Hallow's effect does not mean it happens at that time.  Other players still make choices.  If Player B chooses Hostility's effect to prevent damage (by choosing to apply it first) then Hallow does not have that damage to prevent anymore.

@OrigamiRobot, If Player B is AP, and chooses to apply Hostility's effect to prevent damage, he only does so for the damage for which he is the affected player. You are correct that at that point, he cannot choose to apply Hallow's replacement effect, because he is no longer has any relevant replacement effects for which he is the affected player/object's controller. I will edit my answer to correct this.


text:
What happens when you remove a token creature from the game, and then return it under it's owner's control?

Player A plays Roar of the Wurm
Player B plays Astral Slide
Player B recycles a card and returns the Wurm token

What happens then?

Tokens disappear whenever they leave the battlefield. The relevant rules are:

110.5f A token that's phased out, or that's in a zone other than the battlefield, ceases to exist. This is a state-based action; see rule 704. (Note that if a token changes zones, applicable triggered abilities will trigger before the token ceases to exist.)
110.5g A token that has left the battlefield can't move to another zone or come back onto the battlefield. If such a token would change zones, it remains in its current zone instead. It ceases to exist the next time state-based actions are checked; see rule 704.

So the Wurm token would be gone for good in this example.

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text:
Can a well-timed Twisted Image kill a player using Tree of Redemption?
What happens if my opponent activates Tree of Redemption and I cast Twisted Image with the life-exchange ability on the stack?
Looking at the comp rules, these seem most relevant:

701.8a A spell or ability may instruct players to exchange something (for example, life totals or    control of two permanents) as part of
  its resolution. When such a spell or ability resolves, if the
  entire exchange can’t be completed, no part of the exchange occurs.
Example: If a spell attempts to exchange control of two target creatures but one of those
             creatures is destroyed before the spell resolves, the spell does nothing to the other
             creature.
112.7a Once activated or triggered, an ability exists on the stack independently of its source.    Destruction or removal of the source
  after that time won’t affect the ability. Note that some    abilities
  cause a source to do something (for example, “Prodigal Sorcerer deals
  1 damage to    target creature or player”) rather than the ability
  doing anything directly. In these cases, any    activated or triggered
  ability that references information about the source because the
  effect    needs to be divided checks that information when the ability
  is put onto the stack. Otherwise, it    will check that information
  when it resolves. In both instances, if the source is no longer in the
  zone it’s expected to be in at that time, its last known information
  is used. The source can still    perform the action even though it no
  longer exists.

Is the entire exchange ability countered because the Tree is dead by the time it resolves? Or do we use the "last known state" (toughness 0), thereby killing its controller?

There is some possibility for a sneaky trick here if you give the Tree +1/+1 first :-P

Rather interesting question.  To be dastardly with it you'd have to give the Tree +1/+1 first as Alex said, then it'd be 14/1 when the ability resolves.

To continue the digression: it's entirely possible that the Tree would already have been beefed up (e.g. Giant Growth) to make the toughness/life exchange better, so you might be able to hit pretty hard without extra effort.

From the Oracle:

9/22/2011 If Tree of Redemption isn't on the battlefield when the
  ability resolves, the exchange can't happen and the ability will have
  no effect.

Which is consistent with the golden rule that if one rule says a thing can happen and another says it can't, can't wins.

State Based Effects (SBEs) are continuously checked at every sequence of stack/step/phase and when Tree of Redemption's (ToR) toughness == 0, he's no longer around for his ability to resolve legally.

Player plays ToR's ability. Ability goes on stack. (At this time it's a legal ability as there's 13 toughness to the tree.)
But Opponent, in response, plays Twisted Image (TI) targeting ToR. This goes to the top of the stack.
No more fast effects are played and the stack is resolved, where spells/abilities at the top gets to resolve, followed by the next one and so on.
TI's spell resolves, and causes ToR's Power/Toughness to be switched. At this point, SBEs are checked and causes ToR to go to the graveyard (Since it's toughness is 0).
ToR's ability resolves, and at this point, since ToR is already in the graveyard, it's ability is unable to find it's target (which is ToR's toughness). Therefore this ability fizzles and nothing happens. SBEs are checked again and nothing has changed so nothing happens.
Players move on, and the player in control takes the next action/phase in the game.

Sorry for the long-winded explanation, but I hope this clarifies fully on this event.

Great explanation!

Hey, don't be sorry, detailed explanations are what's necessary to clarify why the cards interact that way. :)

Huh? I'm not seeing the word "target" anywhere in Tree of Redemption's rules text. You may want to clarify why the exchange doesn't happen, because that's not why.

State Based Effect is a deprecated term.  It's been State-based Actions for a while now :)

State-based actions happen automatically whenever a player would receive priority. One state-based action is the one that causes creatures with toughness 0 or less to be placed into their owner's graveyard. Also, exchanges can only happen if the entire exchanged can be completed (as per rule 701.8a quoted in the question.) How the entire play scenario goes down is like this, assuming it is a two-player game and it is Player 1's turn and she has priority:

Player 1 activates Tree of Redemption's ability. It is put onto the stack. She then implicitly passes priority by not holding it.
State-based actions are checked, then Player 2 (the opponent) receives priority. He casts Twisted Image targeting the Tree. This goes onto the stack on top of the Tree's ability. He then implicitly passes priority by not holding it.
State-based actions are checked, then Player 1 receives priority. Player 1 passes priority by playing no more spells or abilities, which causes the top of the stack (Twisted Image) to resolve.
Twisted Image switches the Tree's power and toughness.
State-based actions are checked. This causes the Tree to be immediately placed into its owner's graveyard as it has 0 or less toughness.
Player 1 receives priority, then passes it.
State-based actions are checked. Player 2 receives priority, then passes it, which causes the top of the stack (Tree of Redemption's ability) to resolve.
At this point, since the game is no longer aware of the Tree of Redemption object that created the ability and thus there is no toughness to exchange with the player's life total, the exchange specified by Tree of Redemption's ability does not happen, as per rule 701.8a quoted in the question.

"Also, exchanges can only happen if the entire exchanged can be completed" - this is the key point.

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text:
How can I play around targeted discard?
Many decks in competitive Magic have access to targeted discard spells, like Duress and Thoughtseize.
Against counterspells, you can rely on a variety of tricks like baiting counters, bottlenecking your opponent's mana, and good old-fashioned counter wars. Targeted discard cards allow your opponent to attack your hand on the very first turn of the game, and go right for your key cards — and it can often be backbreaking when your opponent takes away a crucial card like a combo piece or a sweeper.
What tricks and tactics can you employ, either during deck-building or in play, to play around an opponent's targeted discard?
(I know that one of the answers is going to be Force of Will. That's excellent, but please also include approaches that work in formats without free countermagic.)

There are quite a few ways to play around discard decks:
Play Leyline of Sanctity to make yourself Hexproof.
Since all relevant discard effects (except combat damage effects) come from spells and abilities that require you, the player, as a target, this one card hardcounters discard decks and, incidentally, also burn and many combo decks.
Play a deck without cards whose loss would cripple your strategy/combo
Sligh comes to mind, or White Weeny, or Stompy. Anything but Combo, basically, preferably Aggro. If there are no key cards in your deck, you can't lose them to discard.
Play a deck that benefits from discard
Nowadays, there are many cards, keyword mechanics mechanics, and deck archetypes that benefit from you having few cards in hand or many in your graveyard. Cursed Scroll, Ensnaring Bridge, Crucible of Worlds, Tarmogoyf, Hellbent, Threshold, and the Reanimator archetype are examples.
Draw more cards than your opponent can make you discard
All card advantage strategies apply. While this does not directly counter targeted discard, it allows you to draw more cards and therefore more copies of your win conditions, overwhelming your opponent's capability to discard them. Of course, if you lose all of your win conditions to discard, with no way to retrieve them from the graveyard, no amount of card draw will salvage the game.
Tutor for key cards
Nearly all discard, and AFAIK all targeted discard has sorcery speed. Use instant speed tutor effects like Mystical Tutor to get you the cards you need exactly when you need them, without an opening for your opponent. That increases your flexibility and allows you to carry 1 or 2 each of a wide range of strong cards.
Use targeted recycle to get the discarded card back
Use cards like Regrowth or Noxious Revival to simply return the card you had to discard. Either you get the card back immediately, or, if you have to put it on top of your library, play the recycler at the end of the opponent's turn. If enough cards are in your graveyard, this even acts as a kind of tutor, since you may get to choose from a variety of cards. Recyclers are not wasted, either, when your opponent doesn't play discard, so they can be easily integrated into the main deck.

There is one thing you can do:
http://gatherer.wizards.com/pages/card/Details.aspx?multiverseid=204993

Great answer! I'd argue that playing a deck like Kithkin, Lava Spike, or Stompy is sort of its own penalty: you get to duck pinpoint disruption (including targeted discard but also, practically speaking, counters and spot removal), but at the cost of a lot of flexibility. And your opponent can just side out (or not side in) the disruptive cards.

The examples you gave for cards that benefit from discard are all cards that you still need in hand/in play, but like other cards to be in your graveyard. There are also ones that are even fine if they get discarded. There's really obvious ones like Loxodon Smiter, but also things like flashback spells (discarding them only takes half the use away).

To supplement Hackworth's excellent answer, I want to expand a little bit on why card draw in particular can be powerful against discard strategies.  Discard is inherently linear, in the sense that one discard spell doesn't make another inherently better; each Thoughtseize will trade one-for-one with the best card in your opponent's hand, but they don't interact much with each other and in extreme cases they can even interfere - stripping three cards from your opponent with one Fugue, for instance, might mean that the next one you cast only catches one card unless you wait a couple of turns.  By contrast, draw spells have a tendency to cascade; drawing additional cards means that you have more opportunity to draw additional card-draw spells, which in turn will feed even more cards into your hand.  This is somewhat less true with targeted discard, which is a great part of the reason why those spells tend to see play while 'victim-chooses' discard seldom does; and the cascading effect is somewhat less true with 'cantrip' card-draw, which I suspect is why Wizards has made those the primary form of efficient draw spells lately.

This is a good observation. I think it's useful to point out that "real decks" don't try to bury you in targeted discard, though, just like most control decks aren't wall-to-wall permission. Instead they focus the disruption on taking out whatever's situationally most powerful, such as sweepers or powerful hate cards.


text:
What happens when the {X} in Mind Grind *is* zero?
Normally, when you cast a spell without paying its mana cost, the {X} in its mana cost is 0.
Mind Grind, however, has an additional restriction:

X can't be 0.

Clearly you can't choose for {X} to be zero when you cast it normally. But what happens if you don't get to choose, e.g. when you cascade into Mind Grind or cast it with Epic Experiment?

Curious, why the need for the X can't be 0 restriction? It seems like if that weren't there, and you did cast it for x=0, then each opponent simply wouldn't have to reveal any cards, because they have already "revealed 0 land cards". So it would be a pointless, wasted spell... why the additional restriction?

@Gendolkari I think the restriction is there because the card would be confusing without it. The card says "Each opponent reveals cards from the top of his or her library until…" If X is 0 do they still reveal the top card? Rewording the instructions to be clear is harder than simply adding a restriction.

You must choose zero for X, but you can't choose zero for X, so you can't cast the spell. Epic Experiment doesn't force you to cast anything ("may cast"), so it will be moved to the graveyard by the later part of Epic Experiment's effect.

107.3b If a player is casting a spell that has an {X} in its mana cost, the value of X isn’t defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0. This doesn’t apply to effects that only reduce a cost, even if they reduce it to zero. See rule 601, “Casting Spells.”

101.2. When a rule or effect allows or directs something to happen, and another effect states that it can’t happen, the “can’t” effect takes precedence.

601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. Casting a spell follows the steps listed below, in order. If, at any point during the casting of a spell, a player is unable to comply with any of the steps listed below, the casting of the spell is illegal; the game returns to the moment before that spell started to be cast (see rule 717, “Handling Illegal Actions”). Announcements and payments can’t be altered after they’ve been made.

You can see these rules applied by an official source in the Oracle rulings for Ertai's Meddling:

10/4/2004     Ertai's Meddling can't be cast through any way that doesn't
pay its mana cost. This is because the X in the Meddling's mana cost
can't be 0, but effects that allow spells to be cast without paying
their mana costs set X to 0.

Excellent answer - +1 for the rules quotes.

(Even if Epic Experiment didn't use "may", you still wouldn't cast the spell since that would be illegal. As always, you would do as much of the effect as possible, which would be everything but cast this spell.)

To be clear, if you cascade into Mind Grind, the card would remain on top of the library because of 601.2? You're not allowed to "cascade past it"?

@corsiKa, Bottom. "Then put all cards exiled this way that weren’t cast on the bottom of your library in a random order." It would would indeed block Cascade if the cmc is right.

If you cascade into Mind Grind, you're simply not allowed to cast it.  Cascade is a 'may' effect, and so is Epic Experiment.  The situation is essentially identical to revealing something like Flame Slash (4 damage to target creature) with no creatures in play — since the spell can't be played without a legal target (or in Mind Grind's case, can't be played with an X of 0), it's just never cast.  In the case of Cascade, this means that Mind Grind will be one of the cards put on the bottom of your library; in the case of Epic Experiment, it means it will be put in your graveyard without having been cast.


text:
Do I still get to draw cards if I can't sacrifice Soul Ransom when its ability resolves?
Soul Ransom has an interesting ability:

Discard two cards: Soul Ransom's controller sacrifices it, then draws two cards. Only any opponent may activate this ability.

I'm aware that the normal procedure for resolving abilities is "do as much as you can." The use of "then" in the card text is confusing me a bit, though.
If Soul Ransom is already in the graveyard when its ability resolves, because the affected creature has been killed by some other means, do I still get to draw two cards?
Scenarios where this matters:

I Soul Ransom a Thragtusk. At the end of my turn, my opponent discards two cards to activate Soul Ransom's ability. I have a Devour Flesh in hand, so it'd be real nice to give myself some life and a Beast token instead of just returning the Thragtusk. If I Devour-Flesh the Thragtusk in response, Soul Ransom will end up in my graveyard by the time its ability resolves. Will I still get to draw cards?

I'm playing a control mirror and my opponent Soul-Ransoms my Angel of Serenity. He only has 10 cards left in his library. Can I Sphinx's Revelation for 12, then put Soul Ransom's ability onto the stack multiple times to force him to draw himself to death? (Even though only the top-most instance will cause Soul Ransom to be sacrificed.)

Same question on Wizard's Rules Q&A forum.

Yes to all three questions.

112.7a Once activated or triggered, an ability exists on the stack independently of its source. Destruction or removal of the source after that time won’t affect the ability.

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

If the draw was contingent on the sacrifice, the sacrifice would be made part of the cost, or the effect would be "Soul Ransom's controller sacrifices it. If he does, he draws two cards". See Garruk, the Veil-Cursed's second ability for an example of this.

@Colin D, yeah, asking yourself what the wording would be if the card behaved differently is a good way of making sure an answer is correct. It also helps you find counter examples such as Garruk.


text:
What's actually good against Jund?
"Jund" is the red-black-green shard from Shards of Alara.
More importantly, however, it's a term for a midrange archetype in those colors, using a mixture of quick disruption, multi-faceted removal, and powerful creatures, with lots and lots of 2-for-1s, via cards like Bloodbraid Elf, Hymn to Tourach, or Thragtusk. This is clearly a powerful strategy since we've seen periods where Jund decks were ascendant in Standard, Extended, Modern, and Legacy — often multiple formats at the same time!
Magic pros have said things like, "Jund is at best 55% and at worst 45% against everything." Still, having a "55%" matchup against Jund is much better than a "45%" matchup; and when Jund decks are powerful, it's useful to at least have a good sideboard plan against the deck.
For example, the Jund Standard decks at Pro Tour Gatecrash featured these elements:

Hand disruption: Liliana of the Veil, Rakdos's Return, Duress
Resilient threats: Thragtusk, Garruk, Primal Hunter
Powerful removal: Dreadbore, Abrupt Decay, Victim of Night, burn spells
Plentiful sources of card advantage: Thragtusk, Huntmaster of the Fells, Olivia Voldaren, Mizzium Mortars, Rakdos's Return
Incidental lifegain on cards that are great even without it: Thragtusk, Huntermaster of the Fells
Aggressive utility lands: Kessig Wolf Run (compare to the plentiful use of manlands in other formats)
Occasional mana acceleration: Deathrite Shaman, Farseek, Arbor Elf

What kinds of cards and strategies are particularly strong against Jund?
("Jund" is a broad category of decks, but it is a very clear archetype. Feel free to focus your analysis on one format, e.g. Standard.)

I'm not sure about this, so I'll just write a comment. The weak point of Jund seems to be the mana base. So attacking it could be a good strategy.

@Pablo The problem with 'attacking the mana base' at least vs. Standard Jund is that aside from Ghost Quarter (which seems like it's being underplayed in general right now, admittedly, but does have some issues - especially since most Jund decks still run a couple of basics to blunt its impact), there's no really meaningful land destruction in the format right now. By the time you're casting Demolish on the draw they've already played Arbor Elf/Farseek into Huntmaster/Olivia into Thragtusk.

Jund is the prototypical midrange deck - serving as a control deck in the matchups vs. aggressive decks while playing a more aggressive role itself vs. 'true' control decks.  Unfortunately for the Jund deck, it's difficult to do both of these things at once - generally decks of this kind want to be tuned for a particular environment, and guessing wrong can be a problem for the deck.  What's more, while resilience is its strong suit, there isn't any one thing it does better than any other deck can do it, and so it's vulnerable to both ends of its range.
This points to the core ways of dealing with it (note that I'm primarily focusing on Standard here as it's what I'm most familiar with, but most of the core concepts here should apply to other formats) :

Go under it.  A deck like the Saito or Christiansen G/R decks or the 'aggro Jund' decks (which are more 3-color decks that happen to be in the appropriate colors than the sort of resilient Jund deck this post talks about) can easily kill before Jund has a good chance to establish itself, and often build enough board presence early that by the time the Jund player hits 5 mana and starts running out Thragtusk the aggro deck is able to kill them around it.  This seems to be the preferred approach at the moment, judging by the results of the Quebec City Standard Grand Prix where the top 8 was flooded with more aggressive decks; but much of this may be a result of the Jund decks tuning themselves more against the control and midrange (including mirror) matchups than against the aggro decks (for instance, the rising numbers of Olivia Voldaren in Jund lists); I would expect this plan of attack to have more problems as the Jund decks start adapting with e.g. more Vampire Nighthawks.
Go over it: this is the typical control approach, of course.  Right now it might be more accurate to call this plan the 'go around it' plan — the goal is to be a slower, more powerful deck than Jund by playing on axes where Jund has a harder time interacting.  The Nephalia Drownyard/Jace, Memory Adept mill plan is the perfect example of this — rather than trying to beat Thragtusks or Olivias in a fair fight, the Esper control decks just ignore them outright and go for the library.  Backed with cards like Witchbane Orb to keep the Jund deck from attacking spots where the control decks are actually vulnerable, this strategy can be a trump; typically most Jund decks have no actual defense against a mill plan.
Another way of "going around it" is via combo — since Jund has no way (in the main deck, and very few in the board) to actually prevent its opponents from resolving spells, a deck that can win the game from any board position with a single resolved spell is a difficult matchup.  The prototypical example of this in Standard are the combo reanimator decks like Tzu Ching Kuo's from the Quebec City GP; even having its Unburial Rites discarded by Liliana or Rakdos' Return isn't a major problem, since that deck is just as happy to cast it out of the graveyard and win from there.  Birthing Pod decks in Modern work similarly, since just a single resolved Birthing Pod will generally overwhelm the Jund deck with too much card advantage to be effectively dealt with — any creature becomes a serious threat at that point.

On #2: Well, you can only ignore them for so long, on account of life totals. "Side-step their lifegain and board position" makes sense, but you still won't live long enough to beat them unless you can reliably deal with big beaters or Garruk pooping out a dozen Wurm tokens.

@AlexP True enough - but that's generally what the Azorius Charms and Supreme Verdicts are for.  The point is that by taking that angle of attack you blank many of Jund's cards - Thragtusk is 'merely' a 5/3 that leaves a 3/3 behind; Abrupt Decay, Victim of Night, etc are near do-nothings (and Dreadbore is often virtally dead); etc.


text:
What is the converted mana cost of a spell with {X} when cast with the Miracle mechanic?
Say I topdeck Entreat the Angels and cast it for its miracle cost of XWW. But my opponent wants to counter it with a CMC-based effect, such as Spellstutter Sprite or Spell Burst.

202.3. The converted mana cost of an object is a number equal to the total amount of mana in its mana cost, regardless of color....
202.3b When calculating the converted mana cost of an object with {X} in its mana cost, X is treated as 0 while the object is not on the
stack, and X is treated as the number chosen for it while the object
is on the stack.

If I spent 5 mana to cast Entreat the Angels with X=3, what is the CMC of Entreat the Angels on the stack?
Do I get to double-count the X I spent because the card's base mana cost is XXWWW?
Or is X "zero" because I'm not actually paying the X in the mana cost?

The mana value (formerly Converted Mana Cost/CMC) is 9.
The XX in the mana cost and the X in the alternative cost are related. In your example, you chose X = 3, so the mana value becomes 3+3+3 = 9. You only pay 3WW, but spells asking for the mana value of an object don't care about what you have actually paid, they only care about what's printed in the upper right corner of a card.
How to calculate the mana value:

202.1. A card’s mana cost is indicated by mana symbols near the top of the card. (See rule 107.4.) On most cards, these symbols are printed in the upper right corner. Some cards from the Future Sight set have alternate frames in which the mana symbols appear to the left of the illustration.
202.3. The converted mana cost of an object is a number equal to the total amount of mana in its mana cost, regardless of color.
202.3b When calculating the converted mana cost of an object with {X} in its mana cost, X is treated as 0 while the object is not on the stack, and X is treated as the number chosen for it while the object is on the stack.

Miracle is an alternative cost, and an object's mana value is not affected by its alternative costs:

118.9. Some spells have alternative costs. An alternative cost is a cost listed in a spell’s text, or applied to it from another effect, that its controller may pay rather than paying the spell’s mana cost. Alternative costs are usually phrased, “You may [action] rather than pay [this object’s] mana cost,” or “You may cast [this object] without paying its mana cost.” Note that some alternative costs are listed in keywords; see rule 702.

118.9c An alternative cost doesn’t change a spell’s mana cost, only what its controller has to pay to cast it. Spells and abilities that ask for that spell’s mana cost still see the original value.

The X in the alternative costs is the same X as those in the mana costs:

107.3i Normally, all instances of X on an object have the same value at any given time.

Isn't that covered by

117.9c. An alternative cost doesn't change a spell's mana cost, only what its controller has to pay to cast it. Spells and abilities that ask for that spell's mana cost still see the original value.

?
So while your Entreat's on the stack, its CMC would be 9 (since CMC is based on the mana cost).

I clarified the question slightly.

@AlexP The rules you yourself cite are clear that the cmc of spells on the stack includes all X mana used to cast it. Seems to me just because you're using an alternate cost to reduce the CMC obviously doesn't change the actual CMC seen by the countering card. Basically you're defining the cmc as 9, but "chosing" to cast it for 5 as an alternative cost.

@Circeus He doesn't define the CMC as 9. The XX in the mana costs and the X in the alternative costs are completely unrelated.

I see where you're coming from, Circeus, in that it seems "fair" for the X in the Miracle cost to be somehow connected the X in the mana cost.  But unless this was explicitly stated in the rules somewhere, we aren't allowed to assume it; and I'm pretty sure that it would be too complicated and fiddly for Wizards to make a rule out of anyway.

@thesunneversets They could have easily connected the regular and miracle costs, by implementing Miracle as a cost reduction effect. They could have said, "you may cast this card when you draw it if it's the first card you drew this turn. If you do, its cost is reduced by (some cost reduction).", something along those lines. Then you would have to pay its actual mana cost, except reduced, and things like Spell Burst would take into account whatever you chose for X.

@Hackworth, hmm, yes, I guess so, for some definition of "easily".  They do like to cut down on wordcount, and it's nice to retain the future possibility of Miracle costs that aren't quite simple reductions.  How could they have comprehensibly phrased this card, for instance: "cost is reduced by XW"?

@thesunneversets I agree that the alternative cost route is vastly more elegant, but that wasn't the question. The (rephrased) question was, "is it possible to design the Miracle mechanic in such a way that the CMC reflect your choice of X, without adding fancy new rules", and the answer is yes, since it has been done before: see Convoke, Affinity etc. Nothing logically stops them from reducing a XX cost by X, except that it would probably be confusing and inelegant. But that's a different aspect.

Circeus, I'm giving you a +1 for this answer too, in recognition of your intuitions being right long before the rest of us got there!

@thesunneversets In retrospect what's missing from my answer was a reference to 107.3g. It seemed so intuitive to me (though I think I'd seen it before while browsing for a different ruling) that it just didn't cross my mind to look it up.

This was bugging me sufficiently that I asked it as a question on the MTG-RULES mailing list, and received the following response (which fortunately confirms the above answers!):

The converted mana cost is 9.
The idea is that you choose X before you determine how much mana you
  pay. You don't say "cast it for 3WW" then attempt to determine the
  value of X, but rather decide on a value for X, announce you're using
  the Miracle alternate cost, and only then determine that, e.g., it
  costs 3WW for X = 3.
X has indeed the same value on every instance on a card, and has the
  value chosen for it when it's on the stack. If you chose X = 3, then
  the converted mana cost is 9 on the stack.

So, to paraphrase it in a way that helps me understand, "the X" of a spell is essentially a feature of a spell that you declare as you announce that spell, in much the same way as its target, or the mode of a modal spell.  It's not so much part of the mana cost, as something the mana cost is later derived from!

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


text:
What happens when I remove Mistbind Clique with its champion trigger still on the stack?
So, this recently happened to me in playtesting:
It's my upkeep. My opponent casts Mistbind Clique.
I don't have a relevant counter, but I do have Path to Exile. He has multiple fairies on the field, as well as enough mana to animate his Mutavault, so I can't get a two-for-one by Pathing anything before the Clique resolves. I let the Clique resolve.
Mistbind Clique's champion trigger goes on the stack. In response, I Path the Clique before its champion trigger resolves.
Path resolves.
What happens when the Clique's champion ability resolves after it's already been exiled?
My understanding is that my opponent still gets the option to exile a faerie (which would stay exiled, since the Clique has already left the battlefield). If he does so, though, does the Clique's tap-down ability apply?

When a Faerie is championed with Mistbind Clique, tap all lands target player controls.

This is a really good question, I'm going backwards and forwards on what I think the answer is!

You are correct in thinking your opponent still gets the option to exile a faerie. However, if he/she does so, the Clique's tap-down ability does not apply.
Mistbind Clique's ability reads "When a Faerie is championed with Mistbind Clique, tap all lands target player controls." The when there implies that it is a triggered ability:

112.3c Triggered abilities have a trigger condition and an effect. They are written as "[Trigger condition], [effect]," and begin with the word "when," "whenever," or "at." Whenever the trigger event occurs, the ability is put on the stack the next time a player would receive priority and stays there until it's countered, it resolves, or it otherwise leaves the stack. See rule 603, "Handling Triggered Abilities."

Since Mistbind Clique is no longer present on the battlefield when the Champion triggered ability resolves, the tapping triggered ability will not trigger.

Out of interest, what sort of ability on Mistbind Clique (other than a triggered ability) would enable the tapping to take place?  Given the way Champion is (key)worded, I'm having trouble visualising any way a subsequent ability could be worded to allow it to still take place, given that the Clique has been removed in response to the Champion triggered ability.

Aha, so the crux of the argument is that they're entirely separate abilities? That's definitely borne out by the formatting of the card.

@thesunneversets If it were actually part of the Champion ability, that would work - like "Champion a Faerie (When this enters the battlefield, sacrifice it unless you exile another Faerie you control. When this leaves the battlefield, that card returns to the battlefield.) If you do, tap all lands target player controls."

Makes sense! Ta!

+1 for a correct answer that was much more confident than my somewhat cautious attempt... which I've deleted accordingly!

@thesunneversets Your latest wasn't wrong by any means! Just seems like the core confusion was more related to the triggered ability than the champion. :)

You said yourself, you Pathed the Clique before its champion trigger resolves, so it would make little sense to assume that your opponent still exiles another faerie.
If your opponent had exiled another faerie, then of course the tap-down ability would apply, but since the champion trigger didn´t even resolve, there also is nothing to tap-down - simple as that.

I agree that exiling a Faerie to the champion trigger after the Clique has left the field would be a 2-for-1. In the context of the original scenario, though, it might still be worthwhile if you got to use the tap-down ability (in the original situation, tapping down all my lands with Mistbind was the only way for my opponent to avoid a soft lock long enough to kill me with dudes -- of course, as Ian wrote, it doesn't actually work that way).

The Champion trigger does resolve. The Mistbind just isn't on the battlefield  as a permanent, so we don't have a card to read the ability from. It was there as a spell on the stack, then a permanent, but as we are resolving the Champion ability it is no longer present.

You could use the ability if you had something else that benefits from creatures being exiled.  (-1) vote for incorrect.


text:
What happens when an Undying creature goes to the graveyard from the deck or hand?
What happens when you discard a creature(with Undying) from your hand to the graveyard? Does it go to the Battlefield (without summoning sickness)?
And what happens if I use Mulch and put all Undying creatures in the graveyard? The same?

Good question, welcome to the site! :D

No, sorry.
The fine print on the cards tell us what we need to know:

Undying (When this creature dies, if it had no +1/+1 counters on it,
  return it to the battlefield under its owner's control with a +1/+1
  counter on it.)

"Dies" refers to "goes to the graveyard from the battlefield." Thus, a creature card that is placed into the graveyard from any other zone doesn't trigger undying.
Since Undying doesn't say anything specific about "summoning sickness" or haste, the regular rules apply. Namely:

302.6. A creature’s activated ability with the tap symbol or the untap symbol in its activation cost can’t    be activated unless the
  creature has been under its controller’s control continuously since
  his or her    most recent turn began. A creature can’t attack unless
  it has been under its controller’s control    continuously since his
  or her most recent turn began. This rule is informally called the
  “summoning    sickness” rule.

It doesn't matter where the creature came from: if it didn't start the turn no the field under your control, it still has "summoning sickness".

Thank you for anwsering and explain me so well! Big thanks! I think now I have to think again about some ideas on the deck I was making. :(

Also, welcome to the site.

You might also add that going to any zone from the battlefield except the graveyard doesn't constitute dying as well.  For example, exiling a creature wouldn't allow it to come back with the counter.


text:
Are cards that interact with sacrificing rare, and why?
For independent reasons, I was looking recently for cards that interact with Sacrificing. At the time (owing mostly to my lack of knowledge in MtG templating), I found none. Looking at the Avacyn restored preview card Sigarda, Host of Herons brought back the question, which was further piqued when I noticed that another preview cards is intended to "mess with" sacrificing (Angel of Jubilation).
Looking through Gatherer (with the above caveat), I found only a few cards cards (Sigarda's predecessor Tajuru Preserver, Dragon Appeasement, Furnace Celebration, Savra, Queen of the Golgari, Mortician Beetle, Thraximundar) with sacrifice trigger effects. I found no card with trigger effects when they are sacrificed to other cards (something which, coming fro a YGO background, I am used to).
Was my research too incomplete or are there really just few cards that interact with Sacrificing other than as a cost or effect?
If there are so few, do you see any design-related reason for it? I mean, given the prevalence of effects triggering by entering the battlefield, drawing cards, creatures dying, attacking, blocking/being blocked etc., it seems strange that such a prevalent game action as sacrificing would have been ignored.

It's not clear what you include in your definition of "interact with sacrificing." The question could be "Why are sacrifice effects so common in Yugioh?" Maybe you could describe how Yugioh sacrifice effects work for Magic players who are not familiar.

@Ian Pugsley Sacrificing and causing sacrifices are clearly not within the ambit of what I mean, these are cards thaat CAUSE sacrifice, not cards that prevent (are there cards that say "you cannot Sacrifice {cardname}"?) or trigger when something is sacrificed in general (as the cards I cite).

@Circeus I mistook what you were asking (in that Sigarda and Angel of Jubilation are fairly unrelated, other than making you think about sacrificing).

You said:

I found no card with trigger effects when they are sacrificed to other cards

In a design article, Mark Rosewater said:

Originally, Perilous Myr dealt 1 damage when it was put into the
  graveyard from play but 3 damage if it was sacrificed. And it wasn't
  the only artifact creature with this rider. As I explained in parts 1
  and 2, black and red had a sacrifice theme that was a little stronger
  in the design handoff. The biggest problem with the rider was the
  phrase "When CARDNAME is sacrificed" led many players to believe that
  they were allowed to sacrifice it whenever they wanted. The
  development team decided that having a death trigger was cleaner and
  still would be slightly better in a deck capable of sacrificing its
  artifacts.

So Mark Rosewater considers sacrifice triggers of that kind bad design. Not that it couldn't work, just that the benefit of putting them into a set is not worth the complexity. Triggering when a card goes to the graveyard is just close enough in most cases that the sacrifice effect is not worth the potential confusion.

Thanks, that does go to answer part of the question (though I must say I think the team gives too little credit to players there IMO).

@Circeus: Funnily enough, I read your comment and then went on to read today's Latest Developments. http://magic.wizards.com/en/articles/archive/latest-developments/my-magic-origin-story-2015-06-05 ; the section "RULES" is very relevant to the idea that "we need to give the players more room to be confused"

The main reason is that "when something is sacrificed" or "when something is exiled" are mechanics that pigeonhole a card into a very narrow range of decks.
Cards that can sacrifice themselves (as part of the cost for an activated ability) are much easier to use than cards that do something when sacrificed but require an external ability to do it.
Alternatively, a simple death trigger will also work with sacrifice without restricting the card so narrowly. (You can also uses "leaves the battlefield," if you want to trigger on exile or return-to-hand effects as well.) This way, a card that works optimally as fuel for a sacrifice engine can also be used in other circumstances, e.g. as a value-added chump blocker. Broadening synergy is particularly relevant in Limited (i.e. draft), where players won't necessarily be able to play the 2-3 cards that go best together in the same deck all the time.
The major exceptions are cards designed to be the lynchpin of "Johnny" decks that would be, well, "too good" if they were based on straight-up death triggers. This is where cards like Savra fit in.
Beyond that,

Alara block had the devour mechanic, which was usually rather weak outside of a few cards. Dragon Appeasement offers a way to at least make those decks fun in a casual environment.
Rise of the Eldrazi has the annihilator mechanic, which Tajuru Preserver defends against. Mortician Beetle is designed to interact with all the self-sacrificing Eldrazi Spawn tokens in the set.
Scars-Innistrad Standard involves a lot of hexproof creatures, which are immune to spot removal. Effects that force an opponent to sacrifice a creature (commonly called "edicts" by long-time players) can get around that immunity. Sigarda shuts down that plan and forces opponents to use blockers and sweepers to deal with your creatures.

In part, you're seeing this design space explored more now because Magic's M10 rules changes, which removed a timing interaction that allowed players to sacrifice a creature while still trading with an opponent's creature in combat, have allowed designers to push the power of sacrifice effects.

"Mechanics that pigeonhole a card into a very narrow range of decks." Given that such cards have been made in the first place (albeit in sets that did include a high number of sacrificing cards 30+, whereas Innistrad for comparison has 18) goes rather strongly against that argument.

@Circeus Dragon Appeasement is a card you can draft around. If it doesn't work out, it's only one card in your pool you don't have to play. In contrast, if all the supporting cards for it said "When this is sacrificed" rather than "Sacrifice this: do a thing" or "When this dies, do a thing," you'd be stuck with a rubbish deck that just doesn't do anything.

"Magic's M10 rules changes, which removed a timing interaction that allowed players to sacrifice a creatuer while still trading with an opponent's creature in combat, have allowed designers to push the power of sacrifice effects." I had seen the change (I find rule evolutions to be fascinating, and admittedly could simply not comprehend how the eliminated manipulations worked), but I wouldn't have expected this was a consequence. I had noticed these sets had a lot of sacrificing, but not made the connection with specific mechanic (my lack of practical knowledge shows here, sorry...).


text:
Multiple effects causing extra phases in M:tG
Just checking that I understand rule 500.8 correctly:

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.

If multiple effects add extra phases before the current ones is over, these phases will run one after the other, right? If Medomai the Ageless attacks alone while Finest Hour is on the battlefield and then deals damage, I'll get two extra combat phases?

ETA:
Obviously I had a big brainfart, but I guess the question was answered anyway. For a more practical example with extra turns, it's safe to assume that if I connect with both Medomai and Wanderwine Prophets (and sacrifice a merfolk), I'll get two extra turns?

Did you maybe mean something like Aurelia? Medomai gives an extra turn, not a combat phase.

Medomai adds turns, not phases, so the rule you quoted isn't really necessary. Assuming no blockers, here's what happens:

Medomai attacks on your turn. Finest Hour's abilities trigger.
You can stack Finest Hour's abilities in either order. Assuming no other actions, Medomai is now 5/5 and there is now a combat phase after the current combat phase.
Medomai deals damage. Medomai's ability triggers, so you take an extra turn after this one.
You get another combat phase. Medomai attacks again, gaining another Exalted trigger.
Medomai deals damage. Medomai triggers and you get another turn after this one (and before the other extra turn, not that it matters)
You take two consecutive turns in which Medomai can't attack. Each of those turns can get a bonus combat phase from Finest Hour if you have non-Medomai creatures to attack with.

The reason that the placement of the second extra turn matters is that an opponent could steal control of or more commonly  copy Medomai's triggered ability. I can't think of a card that does this off of the top of my head, though.

@John well yeah, basically anything could matter if you stipulate the presence of a card that will make it matter. A more pertinent example might be if someone activates Mindslaver between the two combat steps.

On the contrary, anything could not matter if you stipulate the absence of relevant cards from your opponent. Rather than add another assumption to the beginning of a false statement to make it true, why not remove the false statement?

@John: It's a reasonable default assumption that cards not explicitly mentioned are absent. Last I checked the test to become a judge operates under this assumption.

Last I checked, there is no question on the judge's test like "You have an Elvish mystic on the board and your opponent is at 20 life. Do you win?" The absence of relevant cards does not imply absolute uselessness of placing extra turns in the correct order. Elvish Mystic wins games single-handedly, assuming you have nothing relevant to play. Is this information useful? No.

@John: As I recall, however, there were plenty of questions along the lines of "You have X Grizzly Bears in play and your opponent casts Pyroclasm. How big is your Lhurgoyf?" And no one feels the need to preface every "You would win the game" with "unless you have Abyssal Persecutor" in play. The specific cards mentioned to not care in what order OP takes the extra turns. And I even specifically mentioned what order the turns are in despite it not generally mattering, so the reader can easily extrapolate for a different game state when it does matter.

Next time I am taking a judge test and I answer the question "You have X Grizzly Bears in play and your opponent casts Pyroclasm. How big is your Lhurgoyf?", I will respond, "My Lhurgoyf is (X)/(1+X), not that it matters." I hope I get bonus points for actually answering the question, despite it generally not mattering.

@John In this case the order of the 2 turns is irrelevant. It has no outcome on the game state absent some intervention. In the example you just gave, the power of the creature is always a relevant part of the game state.

@Cruncher You are wrong. Changes in power or toughness are not a relevant part of the game state. To quote an actual judge, "Visible game state includes life totals, the zone of any object, whether an object is tapped, but does not include giant growth type effects, nor abilities that a creature may lose or gain." - fistofthor, LV1 Judge (http://www.reddit.com/r/mtgjudge/comments/1a9m5o/thinking_thursday/)

@John the question is EXPLICITLY asking you for the p/t of the Lhurgoyf. How can it not be relevant to that question? that IS the question.

@Cruncher I was simply applying Chad Miller's logic to other situations. According to Chad, in the absence of a card that makes it relevant, the power and toughness of a creature must not be relevant. We are saying the exact same thing, except that YOU both feel the need to put "it doesn't matter" next to everything that doesn't matter, while I feel that it is extraneous and inconsistent with the remainder of the answer.

@Cruncher For example, "You can stack Finest Hour's abilities in either order". Why not tack on a "not that it matters" clause? Absent some intervention, it does not matter, and yet we didn't really have to say that, did we?

@John: "YOU both feel the need to put "it doesn't matter" next to everything that doesn't matter" Now you're just making things up.

@ChadMiller Which part was made up?

Order of extra turns matters if not all of your extra turns are the same, such as the extra turn created by Final Fortune (which, unlike a regular extra turn, you lose the game at the end of).


text:
How does the Miracle mechanic interact with Flashback?
The new Miracle mechanic causes a replacement effect for the casting cost of the card, which must be revealed and played immediately. How does this interact with flashback? If a player plays a card for its Miracle cost, what is the flashback cost of the card if it's flashed back with something like Snapcaster Mage?

Snapcaster Mage: Bold for emphasis.

Flash
When Snapcaster Mage enters the battlefield, target instant or sorcery card in your graveyard gains flashback until end of turn. The flashback cost is equal to its mana cost. (You may cast that card from your graveyard for its flashback cost. Then exile it.)

The flashback cost will be the mana cost of the card, shown on the top right corner of the card. Alternative costs such as the Miracle cost are irrelevant.
But what if you had a Bösium Strip in play?

Miracle {cost} (You may cast this card for its miracle cost when you draw it if it's the first card you drew this turn.)

The Miracle cost may only be payed if that card is the first card drawn from your library in a turn, and only when you draw the card. Clearly if the card is in your graveyard, it cannot be drawn, so the replacement effect for the casting cost will never apply.
As an aside, note the implications for a card like Future Sight. If you play the top card in your library that's revealed, you won't be drawing it, so you must pay full cost.

"The Miracle cost may only be payed if that card is the first card drawn from your library in a turn. Clearly if it is in your graveyard, it cannot be drawn, so the replacement effect for the casting cost will never apply."... it's still "this card" in the graveyard for the turn you drew it and played it. The question relates to playing it that turn you drew it and flashing it back that same turn. It's still "that card"...

@Hupperware but the Miracle reminder text contains the condition "when you draw it". You cannot pay to cast the card for its miracle cost at any other time.

Furthermore, even if you could "draw it" from your graveyard, at that point it would have no memory of its previous life (see 400.7) and therefore it would no longer be the first card you drew this turn. It would be the second.

Miracle is an ability that only applies as you draw the card. This has no interaction with casting from the graveyard. (I can't speak for future MTR, but I'm assuming judges won't let you pay the miracle cost if you've already let the card touch your other hand cards, period.)
Furthermore, the general rule is that alternative costs don't mix.

117.9a Only one alternative cost can be applied to any one spell as it's being cast.

It's only by paying the alternative cost for flashback that you are allowed to play the spell from your graveyard in the first place; normal cost modifiers (like Thorn of Amethyst) and additional costs (like kicker) still apply, but the base cost is the flashback cost and only the flashback cost, not the miracle cost or madness cost or any other alternative cost. A Mindbreak Trap you've given flashback with Snapcaster Mage, for example, will still cost 2UU to play even in circumstances where it would cost 0 to play from your hand (due to its trap ability).

Wouldn't it be simpler to cover the Snapcaster Mage case by noting that the Miracle Cost is an alternative cost, and not the Mana Cost (i.e. the one covered by Comprehensive Rules 202.1), so it's completely irrelevant to Snapcaster Mage in the first place (and AFAICT all similar cards, which all make the Flashback cost equal to the Mana Cost).

@Circeus It's the most real-world-relevant example of cards with flashback and other alternative and additional costs, since you can apply flashback to any sort of weird card, and Snapcaster is a staple across all tournament formats.

I mean that invoking that "alternative costs don't mix." seems unnecessary since Snapcaster and co. won't "look up" for the presence of any such cost in the first place (as far as I can tell).

@Circeus Good point. That's clearly the most straightforward "NO" answer to Snapcaster Mage. There are truly many roadblocks stopping the Snapcaster Mage from abusing Miracles. :)

@Circeus But "alternative costs don't mix" is the main reason you don't get to reference a card's other costs when flashing it back. Lots of players make the mistake of think that a card's other alternative costs (particularly ones that are phrased like Traps rather than like Miracles, using "instead" or "rather than" to modify its normal mana cost) will also apply to casting it with flashback. And additional costs in the body text -- like Fireball's -- do apply to its total cost when it is cast via flashback.


text:
Can Angel's Grace set your life total to 1 when you have Negative Life?
An opponent plays a storm combo, ending in Tendrils of Agony. In response I play Angel's Grace. After the storm of Tendrils finishes resolving, I am at -10 life, but I have not lost the game because of the Angel's Grace effect. If I take one point of damage (from something like City of Brass).
What will my resultant life total be?

What was your life total before Tendrils of Agony?  How many copies were produced?

After the storm of Tendrils finishes resolving, I am at -10 life, but I have not lost the game because of the Angel's Grace effect.

For a weird case like this, it's best to consult the Oracle rulings.
In this case, we're in luck, since this situation is covered directly:

If your life total goes below 1, being dealt damage will not increase it back up to 1.

So, according to the official rulings, damage from City of Brass can't "reduce" your life total to a higher number than its current value.
So, the remaining question is: does your life total stay -10 or become -11? My best guess (and it is a bit of a guess, to be honest) is that your life total becomes -11 because the "instead" just doesn't apply at all.

It will be -11.  The event isn't "Damage that would reduce your total to less than one" if it's already less than one.  Yes it could be argued in circles for days, but that is the understood interpretation applied by judges.  I think there's an [O] answer somewhere but I'm not finding it in my mailing list archive, might be from before I was a member of the list.

However, this is only until the end of the turn. As soon as the turn ends and the new upkeep starts, you are with one hit point and no more damage reduction effect (i.e. a single damage can kill you).

@Yaztromo You're at -11, so you'd lose the game when state is checked during the cleanup step.

This was asked on the MTGRULES-L mailing list (by one of the people involving in asking/answering this question here? I don't know) and this was the official reply:

An opponent plays a storm combo, ending in Tendrils.
In response I play Angels Grace.
After the storm of Tendrils finishes resolving, I am at -10 life, but
    I have not lost the game because of the Angel's Grace effect.
If I take one point of damage (from something like City of Brass).
    What will my resultant life total be?

-11.
Whenever your life total would change, Angel's Grace applies if your
  previous life total is one or more and your future life total would be
  less than 1. If you were already at less than 1, Angel's Grace doesn't
  apply.
Do keep in mind however that Angel's Grace's effect lasts only until
  end of turn, so unless you raise your life total to 1 or more (or win
  the game, or bring a Platinum Angel onto the battlefield...), you will
  lose in the next cleanup step.
Daniel Kitachewsky L3, Paris, France MTGRules-L Netrep

Just in case anyone was still in any doubt!

@user1873 - I don't believe that's true at all.  See rules 514.2-3a in the Comprehensive rules.  During the cleanup step "until end of turn" effects end, and state-based effects like losing the game are checked subsequently, before the new turn starts.  Though if you're right, then this is a great opportunity to tell a really senior Level 3 Judge off for getting his rules wrong, which would be a lot of fun :D

@user1873. Yes, that's right. But 514.3a explicitly states that state-based actions can be carried out, and players can receive priority, during the cleanup step if circumstances such as this one call for it.

@user1873 Heh! It's weird that "you cannot lose this turn" doesn't mean the same thing as "you won't lose till next turn", so I completely understand where you were coming from.  Magic is a quirky, quirky game.


text:
What effects do exhausted cards have?
In the My Little Pony Collectible Card Game official rulebook, it says that

While a card is exhausted, it can't be involved in faceoffs or help you confront Problems.

This description is a little vague. Does this only mean that the character does not contribute its power to faceoffs and problems? Or do its static abilities also stop working? Do resources on exhausted characters do anything? How about exhausted resources?
In other words, when a card is exhausted, does its text (or the text of attached resources) do anything?

According to the Canterlot Nights version of the comprehensive rules,

(503.5) Exhausted cards do not contribute their power to Faceoffs or to confronting problems.
(702.1) Continuous abilities generate continuous modifiers, and function until the card that has them loses them or leaves the appropriate zone.

So, the abilities keep working.

The key term here is involve. To resolve a faceoff or problem, characters must first be involved in it. An exausted card cannot be involved in a faceoff or problem (except if it is Stubborn) and this applies to resources too. A resource played on a exausted character will not take effect in the faceoff/problem if the character cannot be involved in the faceoff. Only the involved cards contribute to the resolution of a faceoff.

I'm not sure what you mean by "choose the cards that will be involved". The rulebook just says that all characters at the relevant problems are involved. And you didn't cover anything besides faceoffs. It's also still unclear what happens with abilities like "Your other characters at this problem get +1" or "Your opponent's characters at this problem get -1 during the score phase" on a character or on a resource. How about reactions that don't exhaust the character?

Sorry about the usage of "choose". I meant exactly what you wrote in your comment. I'll fix that.

Now, look at the wording of the ability: "Your other characters at this problem get +1". This means that the card that has the ability must first be involved in the problem/faceoff for its ability to affect others. In the manual, page 2, it says "While a card is exhausted, it can’t be involved in faceoffs or help you confront Problems". So, if you cannot involve your character in the problem or faceoff it is not "at it", and its ability won't work.

That seems like a bit of a leap of logic to me. Characters have locations (at one of the problems, or home) independent of the exhausted state, so "not involed in a problem" does not necessarily imply "not at the problem".

Well, I understand that being "at a problem/faceoff" is not exactly the same as "involved in a problem/faceoff", and the rules also say that a character should be "assigned to a problem/faceoff". It seems that the authors allowed some vagueness in the usage of terms that describe the participation of a card in a problem/faceoff. Still, I stick with what I described above because it makes sense to me in game terms.


text:
If creatures enter the battlefield attacking, do they have to attack Gideon Jura?
If my opponent activates Gideon Jura's first ability targeting me, and I attack with a Kessig Cagebreakers, do the tokens have to attack Gideon?

No, the tokens are not forced to attack Gideon the turn they are put on the battlefield. They can attack the defending player or another planeswalker.
Gideon's first ability is a requirement placed on the turn-based action of declaring attackers. That action is the first thing the attacking player do in the declare attackers step.
The tokens created by Cagebreakers are not declared as attackers when the restriction is being applied, therefore they are not affected by it.
See this ruling for Cagebreakers:

Although the tokens are attacking, they were never declared as
  attacking creatures (for purposes of abilities that trigger whenever a
  creature attacks, for example).

And this ruling for Gideon Jura:

The first ability only affects the declaration of attackers. If a
  creature is put onto the battlefield attacking (thanks to Hero of
  Bladehold, Preeminent Captain, or the Ninjutsu ability, for example),
  that creature's controller may choose the defending player or
  planeswalker that it will be attacking in the normal way.

Correct. The tokens weren't around to "attack Gideon Jura if able", so the ability has no effect on them.


text:
What happens if each player has a copy of the same Archetype?
Archetype of Endurance (from Born of the Gods) is a card that says

Creatures you control have hexproof
Creatures your opponents control lose hexproof and can't have or gain hexproof.

If both I and my opponent have one of these in play, then there is one effect that says that my creatures have hexproof, and one that says that they lose hexproof and can't have it. Which effect takes precedence? Does it depend on what order the two creatures came into play? Basically, which creatures, if any, have hexproof while both are in play?

As long as both Archetypes are in play, no creatures have hexproof.
The most important rule to take into account here is rule 112.11:

Effects can stop an object from having a specified ability. These effects say that the object "can't have" that ability. If the object has that ability, it loses it. It's also impossible for an effect to add that ability to the object. If a resolving spell or ability tries to create a continuous effect that would add the specified ability to such an object, that continuous effect is not created, although that resolving spell or ability can still create other continuous effects. Continuous effects created by static abilities that would add the specified ability won't apply to that object.

This works with rule 101.2:

When a rule or effect allows or directs something to happen, and another effect states that it can't happen, the "can't" effect takes precedence.

This means that each Archetype says that its controller's opponents' creatures lose hexproof and can't gain hexproof, and that removes hexproof from those creatures and overrides any ability that would give them hexproof.
If we want to look at exactly how these abilities interact, then we need to consider rule 613: Interaction of Continuous Effects:

613.1: The values of an object's characteristics are determined by starting with the actual object... Then all applicable continuous effects are applied in a series of layers in the following order:
613.1f: Layer 6: Ability-adding effects, ability-removing effects, and effects that say an object can't have an ability are applied.
613.2: Within layers 1-6, apply effects from characteristic-defining abilities first, then all other effects in timestamp order.

The effects created by Archetypes' abilities are not characteristic-defining abilities (according to rule 604.3), so they are applied in timestamp order (i.e. the order in which they entered the battlefield).
If Player A is the player whose Archetype of Endurance entered first, then the effects are applied in the following order:

Player A's Archetype's first ability. Creatures controlled by Player A have hexproof.

Player A's Archetype's second ability. Creatures controlled by Player B all lose hexproof.

Player B's Archetype's first ability. Creatures controlled by Player B would gain hexproof, but the previous ability prevents that from happening (due to the previously mentioned rule 101.2)

Player B's Archetype's second ability. Creatures controlled by Player A lose hexproof.

The end result is that none of the creatures on the battlefield have hexproof. Note that this process of applying effects does not happen at a particular time during the game. Rather, if at any time you want to determine whether a creature has hexproof, this process gives you the answer.

As an aside, upon seeing this card I immediately realized that it appears to be carefully worded so as not to rely on timestamps or anything similarly arcane.

Comprehensive rules are great, but if you just want a quick explanation, the Mechanics of Born of the Gods article answers exactly this question:

... The second ability is worded so that it doesn't matter how or when your opponent's creatures would have or gain flying. Whether they have flying printed on them, gained flying before your Archetype of Imagination entered the battlefield, try to gain it after your Archetype of Imagination entered the battlefield, or are being granted flying continuously (say, by another Archetype of Imagination), they can't have flying for any reason while you control Archetype of Imagination.
This means that if two players each control Archetype of Imagination, none of their creatures have flying.


text:
How do Chromatic Sphere, Archmage Ascension, and Panglacial Wurm interact?
If I have an Archmage Ascension in play with six quest counters and a Panglacial Wurm in my library, and I activate Chromatic Sphere's ability as part of the process of playing another spell, as in rule 601.2f, when do I attempt to start drawing the card? In particular, once the replacement ability takes effect and I instead search my library, can I start casting the Panglacial Wurm while I am still casting the other spell?

can I start casting the Panglacial Wurm while I am still casting the other spell?

Yes. It doesn't matter at all that you are casting another spell[1][2].
If the game instructs you to cast (or gives you to option to cast) a spell, you must (or can) cast the spell. (And that's the only time you can cast a spell.) Most of the time, the instruction is in the rules, but it's on a card in this case.
You might say "But I'm casting a creature when the stack isn't empty, and it might not even be my turn." Doesn't matter. Pangalcial Wurm doesn't say "Cast Panglacial Wurm if it's you main phase and the stack is empty" like priority effectively does[3].

You cast the other spell (TOS):

You place TOS on the stack.
You makes some choices.
You determine TOS's cost.
You activate mana abilities:

You activate Chromatic Sphere's ability.
Chromatic Sphere's resolves immediately:

You add one mana of of the color of your choice to your mana pool.
You search your library for a card to place in your hand:

You cast Panglacial Wurm:

You place it on the stack.
You determine the cost to cast it.
You activate mana abilities.
You pay the cost to cast it.

You place the card in your hand.
You shuffle your library

You pay TOS's cost.

[People get to play spells and activate abilities here if they want.]
Panglacial Wurm resolves:

You place Panglacial Wurm on the battlefield.

[People get to play spells and activate abilities here if they want.]
TOS resolves

Notes:

Chromatic Sphere's ability is a mana ability, and it's possible to activate mana abilities while casting a spell. Casting a spell (or activating an ability) is to put it on the stack, make some choices (including targets), determine its cost, activate mana abilities, and pay its cost.
The stack isn't empty whether you use Chromatic Sphere's ability while casting another spell or not.
Priority could be defined as "Whenever you gain priority, Choose one — Cast a non-instant spell if it's you main phase and the stack is empty; ...; or pass priority unless all players have passed priority in succession."

Did you take this ruling on chromatic sphere into account when answering your this question: This is a mana ability, which means it can be activated as part of the process of casting a spell or activating another ability. If that happens you get the mana right away, but you don't get to look at the drawn card until you have finished casting that spell or activating that ability.  This makes it sound like you do not search until after the spells cost are payed for and the spell is cast.

@Colin D, 401.5 prevents you from looking at cards drawn during casting because a spell can become illegal to cast while it's being cast, and looking would prevent the game from being reverted back to its pre-cast state. It doesn't delay the draw; just looking at the drawn card. I don't think that changes the answer since the rule specifically applies to cards drawn, but I'll confirm with some very knowledgeable people tomorrow.

@Colin D, So far, no discent from my answer. I realize the linked question is different, but I asked that question because I think it has a better chance of validating the answer I asked here than if I had simply re-asked the OP's question.


text:
Does Laboratory Maniac's ability cause me to win before state-based actions are triggered?
The situation: I have 9 poison counters and no card in my library, and I control a Laboratory Maniac.
I cast Steady Progress, and I choose to proliferate the poison counters on me. Do I win the game due to Laboratory Maniac's ability before state-based actions would cause me to lose from having 10 poison counters?

You certainly know this, but for the possible benefit of others: you don't have to proliferate the poison counters to draw a card in this situation.

Should this specify that you have 0 cards in your library?  It seems implied, unless I'm missing something.

@bwarner You're right, I added an explicit mention of that.

You would win because the Maniac's ability replaces the drawing a card with winning the game, so you win before SBA are checked and you lose the game due to having 10 poison counters.

614.1. Some continuous effects are replacement effects. Like prevention effects (see rule 615), replacement effects apply continuously as events happen—they aren’t locked in ahead of time. Such effects watch for a particular event that would happen and completely or partially replace that event with a different event. They act like “shields” around whatever they’re affecting.
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.

Since the card draw is getting replaced with winning the game during the resolution of the spell you will win as the spell is resolving. Since you won the game during the resolution of the spell you never get to the point where the game checks SBAs (including losing the game due to having 10 poison counters)

OK, I thought that's how it worked. I just wanted to make sure I wasn't missing anything.

Is there another rule you can quote that explains that when an effect says "you win", that the game ends immediately, before the next time SBAs are checked?

@GendoIkari There isn't a specific rule that says that, but this works exactly the same as any other effect. You don't wait until SBAs are checked to put a creature that has been Murdered into its owners graveyard, winning the game is no different.

@diego Thanks. I suppose the fact that "check to see if a player has had a 'you win' effect" is NOT in the list of SBAs may be what I'm looking for.

Laboratory Maniac is a replacement effect. The entire effect of Steady Progress has to be carried out, so I believe that you would win before state-based actions would occur.

704.4. Unlike triggered abilities, state-based actions pay no attention to what happens during the resolution of a spell or ability


text:
Does Void Winnower prevent my opponent from casting X spells with even base converted mana cost?
The part of Void Winnower's text that is relevant is

Your opponents can't cast spells with even converted mana costs. (Zero is even)

I know that when checking the converted mana cost of a card with {X} in the cost, the X is considered to be 0 unless the card is a spell on the stack. So Black Sun's Zenith normally has a converted mana cost of 2. Does this mean that Void Winnower completely prevents me from casting Black Sun's Zenith, or that I can cast it if I choose an odd X?

Void Winnower does not prevent you from casting a spell with {X} in the cost as long as you choose an X that results in the CMC of the spell being odd.
Rule 202.3b says

When calculating the converted mana cost of an object with an {X} in its mana cost, X is treated as 0 while the object is not on the stack, and X is treated as the number chosen for it while the object is on the stack.

In addition, the steps for casting a spell contain the following:

601.2a To propose the casting of a spell, a player first moves that card (or that copy of a card) from where it is to the stack. [...]

601.2b [...] If the spell has a variable cost that will be paid as it’s being cast (such as an {X} in its mana cost; see rule 107.3), the player announces the value of that variable. [...]

601.2e Based on the previous announcements, the game checks to see if the proposed spell can legally be cast based on applicable timing rules (including ones based on the card’s type) and other effects that may allow a spell to be cast or prohibit a spell from being cast. If the proposed spell is illegal, the game returns to the moment before the casting of that spell was proposed (see rule 717, “Handling Illegal Actions”).

So, you put the spell on the stack, then you choose X. At this point, the {X} in the cost is equal to the chosen X because of 107.3a. So, when you check whether the spell is legal in the later step, the CMC depends on X, so Void Winnower only prevents it from being cast if the total CMC including X is even.

Void Winnower's Gatherer page now has a ruling that says

For spells with {X} in their mana costs, use the value chosen for X to determine if the spell’s converted mana cost is even or not. For example, your opponent could cast Endless One (with mana cost {X}) with X equal to 5, but not with X equal to 6.

I love how Void Winnower stops Hangerback Walkers.


text:
Can I back out of activating an ability where my opponent chooses the target?
Say I have a Magus of the Arena, and my opponent and I each control other creatures. Magus of the Arena's ability is

{3}, {T}: Tap target creature you control and target creature of an opponent's choice he or she controls. Those creatures fight each other.

Is the following sequence of actions on my part legal?

I declare that I am activating the Magus's ability.
I choose one of my creatures as a target.
My opponent chooses one of their creatures as a target. Specifically, it is a creature I did not want to fight.
I say "I decline to tap my lands for mana" since step 601.2f, only says that I have a chance to activate mana abilies, and not that I must do so. Then I say "I do not have enough mana to pay for the spell" (in step 601.2g).
Now, according, to step 717, we back out of activating the ability, and do something else.

This seems to follow the letter of the rules, but it also seems to violate the spirit of the card in question.

I'm no expert, but to me it seems that to activate that ability you have to spend mana. Spent mana cannot be made unspent (source: my school break knowledge about magic from about 20 years ago)

That's why I'm going into a lot of detail about the steps. Rule 601 has the specific steps for casting spells and activating abilities, and paying costs comes after choosing targets. Plus this is a weird edge case; it depends on applying (or abusing, depending on how you look at it) the rules for undoing illegal actions.

This seems to fall under http://community.wizards.com/content/forum-topic/3703156, since it is effectively the same as saying that you determine a random target and then choose not to pay for the spell if you don't like the random choice.

The difference here is that rule 601.2f isn't relevant in that scenario because that ability had no mana cost. And my scenario partly hinges on the fact that 601.2f does not require you to activate enough mana abilites to cast the spell/activate the ability.

@murgatroid99 The discussion eventually covered a bit more than that, though: "As far as 'announce a spell/ability without the intent to actually produce mana to pay for it hoping to get information from the opponent', this is something that is only technically possible by lawyering the rules, and is far from an intended 'bluff' we want to be available for players. If you try this, you'll get a stern talking to, and you'll stop trying that in the future (or you'll be committing Unsporting Conduct - Major for failing to follow the direct instruction of a tournament official)."

@Jefromi I didn't see that quote in a skim through that thread. Is it from there or somewhere else? But yes, it looks like that question answers this one (or at least refutes a key step).

@murgatroid99 I almost missed it too - it's on the second page of the forum thread.