text:
Which cards count as having been banned or restricted for Spike, Tournament Grinder's tutor ability?
In the new unset, Unstable, Spike, Tournament Grinder can tutor for any current or previously banned or restricted card from outside the game.

Specifically this ability:

ϕϕϕϕ: Choose a card you own from outside the game that has been banned or restricted in a Constructed format, reveal that card, and put it into your hand.

Which cards are on that list, as of the last B&R announcement ( October 17, 2017 )?

First of all, keep in mind that all Un-set questions are always up to interpretation, and may be played different by different playgroups. Although I try to be as precise as possible, due to the lack of official rulings, no definitive answer can be given.
Currently Banned/Restricted Cards
A list of currently banned cards can be found at the official Wizards pages. This is the most readable of all the online sources I'm aware of, and should also be the one that's kept up to date most reliably.
Previously Banned/Restricted Cards in Common Formats
I'm not aware of any official list specifically composed for this purpose, but there are other sources that more or less specific for this purpose will help with the search:

Scryfall (a third-party card search engine) added a custom search tag for Spike-tutorable cards.
The Timeline of Banned/Restricted Cards on the GamePedia MtG Wiki.
A Reddit thread, trying to gather all cards by community effort (Malco's answer provides a copy of that list)

Basically, you would be able to tutor for any card from these lists, provided that the list is accurate.
Previously Banned Cards in Commander and Other Formats
As the previously linked pages don't cover the Commander format, you may resort to the official changelog, which keeps track of bans, but doesn't provide any info prior to September 2009. Whichever logic you apply, going by that list will likely be the most sensible thing to do, simply because of its official origin. That said, there should be other threads you'll find, pointing you towards history further back in time, such as this one that has been suggested in the comments.
Furthermore, there are many more widely played formats (such as Tiny Leaders) I could provide a link to, but the reality is that if you include all those formats in your definition of previously banned, some person will eventually just invent a format that bans every card but basic lands, and therefore will be able to tutor all cards.
A Note on Ante Cards
Ante cards are tutorable, but due to the way they function, they won't reap you any benefits, as confirmed by Mark Rosewater:

Q: Can Spike, Tournament Grinder get ante cards?
A: Ante no longer exists, so they aren’t usable.

In other words, while you can tutor banned or restricted cards, you won't necessarily be able to use them due to the magnitude of changes to the rules over time. Then again, this would also be subject to different interpretations of Un-sets, as stated in the beginning, so your mileage may vary.

One thing that "Previously Banned/Restricted Cards" section doesn't have is previously banned cards in Commander. There's a good forum post here that has a lot of that history

@murgatroid99 That's a great hint, but I think the official changelog is better suited for a quick lookup, especially since it appears to reach further back in time at a quick glance.

The official changelog goes back to September 2009, as you said. The one I linked goes back to October 2002.

@murgatroid99: Whoa, I meant to say "even though", not "especially" in that comment... I even got it right in the edit itself O_o

You say "if your interpretation is that Spike can tutor a card that has been banned or restricted at some point, but isn't anymore", but this isn't up for interpretation, it was explicitly stated

@murgatroid99: Thanks for the hint, I removed that paragraph.

@TheThirdMan One thing I didn't notice in your answer was a mention on un-set cards. Aren't all of these banned in constructed formats? This might be relevant for drafts, where you could tutor up any card you drafted.

@SocioMatt: Not quite. Silver-bordered cards aren't legal to be played in any sanctioned constructed tournaments, but that doesn't mean that those cards are banned from any edition. Bannings and restrictions happen selectively - it's not like all Un-cards are banned, just like not all pre-8th Edition cards are banned in Modern - they're just not legal to play in Modern. As I said, though, this is something that could be house-ruled, although it kind of spoils the fun of the card if you were "just" fetching a card from the current set/draft, in my opinion :)

UPDATE: Scryfall (a mtg card search tool) has recently added the search tag is:spikey. This means you can see a nicely laid out (and searchable) list of the tutorable cards here by searching on their site.
I have left the original Reddit list below as some insurance against link rot.

In addition to TheThirdMan's answer on how to actually find them, there was a list of cards that are currently or have previously been banned and restricted that has been curated by reddit in this thread.
For convenience, and in case the link breaks in the future, here are the cards as listed:
Here are all of the currently banned/restricted cards you can currently tutor up with Spike:

AEther Vial (Mirrodin block)
Aetherworks Marvel
Amulet of Quoz (Ice Age Block)
Ancestral Recall
Ancient Den
Arcbound Ravager (Mirrodin block)
Balance
Bazaar of Baghdad
Biorhythm
Birthing Pod
Black Lotus
Blazing Shoal
Bloodbraid Elf
Braids, Cabal Minion
Brainstorm
Chalice of the Void
Channel
Chaos Orb
Chrome Mox
Cloud of Faeries (Pauper Ban)
Cloudpost
Coalition Victory
Cursed Scroll (Tempest block)
Cranial Plating (Pauper Ban)
Dark Depths
Darksteel Citadel (Mirrodin block)
Deathrite Shaman
Demonic Consultation
Demonic Tutor
Dig Through Time
Disciple of the Vault (Mirrodin block)
Dread Return
Earthcraft
Emrakul, the Aeons Torn
Empty the Warrens (Pauper Ban)
Erayo, Soratami Ascendant
Eye of Ugin
Falling Star
Fastbond
Felidar Guardian
Flash
Frantic Search
Gaea's Cradle (Urza block)
Gifts Ungiven
Gitaxian Probe
Glimpse of Nature
Goblin Recruiter
Golgari Grave-Troll
Grapeshot (Pauper Ban)
Great Furnace
Great Furnace (Mirrodin block)
Green Sun's Zenith
Griselbrand
Gush
Hermit Druid
Hypergenesis
Imperial Seal
Invigorate (Pauper Ban)
Intangible Virtue (Innistrad-Avacyn Restored block)
Jace, the Mind Sculptor
Karakas
Leovold, Emissary of Trest
Library of Alexandria
Limited Resources
Lin Sivvi, Defiant Hero (Masques block)
Lingering Souls (Innistrad-Avacyn Restored block)
Lion's Eye Diamond
Lodestone Golem
Lotus Petal
Mana Crypt
Mana Drain
Mana Vault
Memory Jar
Mental Misstep
Merchant Scroll
Mind Twist
Mind's Desire
Mishra’s Workshop
Monastery Mentor
Mox Emerald
Mox Jet
Mox Pearl
Mox Ruby
Mox Sapphire
Mystical Tutor
Necropotence
Oath of Druids
Painter's Servant
Panoptic Mirror
Peregrine Drake (Pauper Ban)
Ponder
Preordain
Primeval Titan
Prophet of Kruphix
Punishing Fire
Recurring Nightmare
Rishadan Port (Masques block)
Rite of Flame
Rofellos, Llanowar Emissary
Seat of the Synod
Second Sunrise
Seething Song
Sensei's Divining Top
Serra's Sanctum (Urza block)
Shahrazad
Skullclamp
Smuggler's Copter
Sol Ring
Splinter Twin
Squandered Resources (Mirage block)
Stoneforge Mystic
Strip Mine
Summer Bloom
Sundering Titan
Survival of the Fittest
Sway of the Stars
Sylvan Primordial
Temporal Fissure (Pauper Ban)
Thawing Glaciers (Ice Age block)
Thorn of Amethyst
Time Spiral (Urza block)
Time Vault
Time Walk
Timetwister
Tinker
Tolarian Academy
Trade Secrets
Treasure Cruise
Tree of Tales
Trinisphere
Umezawa's Jitte
Upheaval
Vampiric Tutor
Vault of Whispers
Voltaic Key (Urza block)
Wheel of Fortune
Windfall
Worldfire
Yawgmoth's Bargain
Yawgmoth's Will
Zuran Orb (Ice Age block)

And here are all of the previously banned/restricted cards you can tutor up with Spike:

Ali from Cairo (Previously restricted in Vintage)
Ancestral Vision (Previously banned in Modern)
Ancient Tomb (Previously banned in Extended)
Berserk (Previously banned in Legacy)
Beacon of Immortality (previously banned in EDH)
Bitterblossom (Previously banned in Modern)
Black Vise (Previously restricted in Vintage)
Braingeyser (Previously banned in Legacy)
Burning Wish (Previously banned in Legacy)
Candelabra of Tawnos (Previously banned in Legacy)
Copy Artifact (Previously banned in Legacy)
Crop Rotation (Previously banned in Legacy)
Crucible of Worlds (previously banned in EDH)
Cunning Wish (previsouly Banned in Commander)
Dark Ritual (Previously banned in Extended)
Death Wish (previsouly Banned in Commander)
Divine Intervention (Previously banned in Vintage)
Dingus Egg
Doomsday (Previously banned in Legacy)
Dream Halls (Previously banned in Legacy)
Enlightened Tutor (Previously banned in Legacy)
Enlightened Wish (Previously banned in EDH)
Entomb (Previously banned in Legacy)
Emrakul, the Promised End (Previously banned in Standard)
Fact or Fiction (Previously banned in Legacy)
Feldon's Cane (Previously banned in Legacy)
Fluctuator (Previously banned in Standard)
Fork (Previously restricted in Vintage)
Gauntlet of Might
Goblin Lackey (Previously banned in Extended)
Golden Wish (previously Banned in Commander)
Grim Monolith (Previously banned in Legacy)
Grindstone (previously banned in EDH)
Heartless Hidetsugu (previously banned in EDH)
Hurkyl's Recall (Previously banned in Legacy)
Hymn to Tourach
Hypnotic Specter
Icy Manipulator
Illusionary Mask (Previously banned in Legacy)
Ivory Tower (Previously banned in Legacy)
Jeweled Bird (Previously banned in Standard)
Juggernaut
Kird Ape (Previously banned in Extended)
Kokusho, the Evening Star (previously banned in EDH)
Land Tax (Previously banned in Legacy)
Living Wish (previously Banned in Commander)
Maze of Ith (Previously banned in Legacy)
Metalworker (Previously banned in Legacy)
Mind Over Matter (Previously banned in Legacy)
Mirror Universe (Previously banned in Legacy)
Mox Diamond (Previously banned in Legacy)
Niv-Mizzet, the Firemind (Previously banned in EDH)
Orcish Oriflamme
Personal Tutor (Previously restricted in Vintage)
Protean Hulk (Previously banned in EDH)
Rebirth (Previously banned in Standard)
Recall (Previously banned in Legacy)
Reflector Mage (Previously banned in Standard)
Regrowth (Previously banned in Legacy)
Replenish (Previously banned in Legacy)
Rukh Egg
Serendib Efreet
Staff of Domination (Previously banned in EDH)
Stroke of Genius (Previously banned in Legacy)
Sword of the Ages (Previously restricted in Vintage)
Sword of the Meek (Previously banned in Modern)
Thirst for Knowledge (Previously restricted in Vintage)
Test of Endurance (Previously banned in EDH)
Underworld Dreams (Previously banned in Legacy)
Valakut, the Molten Pinnacle (Previously banned in Modern)
Wild Nacatl (Previously banned in Modern)
Worldgorger Dragon (Previously banned in Legacy)

Some cards merged into list based on this mtgwiki page, Commander historic bans, and the pauper ban list.

Upvoted because of its undeniable usefulness for someone, but I'm not keen on this type of post in general. I may not consider Pauper a format to base my evaluation on in a constructed set because I utilize non-common cards, and I may consider Tiny Leaders (which doesn't seem to be included here) because the deck I'm building is a Commander/Tiny Leaders one. Basically, it won't solve the problem for everyone, and might obscure what might be allowed for others. I tried to emphasize the impracticality of additional sets in my answer, but also think the possibility shouldn't be concealed.

@TheThirdMan Your answer is the better one of the two and should remain the accepted one as it shows you how to find the cards for yourself. My answer is more for the lazy who want a quick reference. When adding cards that weren't found in the original Reddit post I tried to include where they were banned so that the lazy could still make their own judgement calls on what is acceptable.

Plus this answer is soooo bloody long, I really wish there was a better way to format this. I don't want it to be accepted as it would push your answer way down and no one would get to see it.

Well, the problem extends beyond my answer really... this way, possible new answers will be looked at less than they normally would have. Personally, I was in favor of the code block formatting from the edit history, even though it does go against its intended use...

@TheThirdMan I really like the scroll bars too, but code blocks wreak havoc with accessibility tools and general readability so it was ultimately decided it wasn't worth it.


text:
Are there 3 or 12 MTG FNM cards for 2018?
Usually FNM issues 12 cards per year. For 2018 we see only 3 cards in that year. Is Scryfall not up to date or has WotC stopped the FNM series?

12, only the 3 double sided tokens were numbered for FNM 2018 the other 9 used their set collector number.
In 2018, Wizards started giving out double sided tokens as FNM promos, this was not a popular decision, so after the first three of those tokens they changed the system. They started giving out versions of cards from the current sets. They had a different frame, promo rarity "P" and the "FNM" name on their collector number line, which had the collector number matching regular pack pulls. Opt was the first one:

The rest were: Cast Down, Shanna, Sisay's Legacy, Elvish Rejuvenator, Militia Bugler, Murder, Conclave Tribunal, Sinister Sabotage, and Thought Erasure. To find all 12 you can use this query on scryfall.
Now, however, they have stopped making specifically FNM promos and instead are having packs of promos using this frame that can be given out by stores how the stores want to.  These promos are marked with the set they are from and numbered as alt art promos are currently, with a just a number higher than the set count, and that count is excluded, They have the rarity "P" and "Promo Pack" for where they came from. At the moment, FNM prizes are digital for MtG Arena, where FNM events are currently played, this may change once the pandemic ends and we get back to playing in store events again.

The FNM list for 2019 is: Growth Spiral, Light Up the Stage, Mortify, Augur of Bolas, Paradise Druid, Dovin's Veto, Corpse Knight, Negate, Disfigure, Flame Sweep, Thrashing Brontodon, Glass Casket, Improbable Alliance, Inspiring Veteran, Kenrith's Transformation, Slaying fire

Some further references: FNM Promo cards, Promo pack cards, and Promo pack cards that are just the inverted frame originals, not including the planeswlaker stamped reprints.

source: Stack Exchange

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

sort: votes


text:
What happens when I Radiate a Time Stretch with Timesifter in play?
The question is based on the following situation:
In a five person game, there has a been a Timesifter in play and the turn order has already been varied for a few turns. A player plays a Time Stretch and I target the card with a Radiate. What happens next, and subsequently, what happens when the person with Timesifter is eliminated from the game?
As an immediate result, I assume that the person who was targeted with the original Time Stretch takes the next turn, which results in a trigger of the Timesifter, sending us back into a whirlwind of turn order that is dictated by the artifact. I'm relatively certain this is correct, although an affirmation would be nice. But, when the person with the Timesifter dies, where does the turn order go? Are the extra turns from Time Stretch still stacked? Does the original target of the spell take their second turn, followed by everyone else taking two extra turns in normal turn order?

There are several different things going on here, so it's important to understand the key details.  Those boil down to:
1) Nothing in your scenario changes the turn order.  This is essential, because it's the first guide to understand what's going on.  While Time Sifter looks like it effectively changes the turn order, instead it specifically generates extra turns itself.
2) This is relevant because the game maintains what amounts to a stack (not to be confused with the actual stack in the game, for resolving spells) of extra turns; any effect that says '~player~ takes an extra turn after this one' adds an extra turn for ~player~ onto that stack.  This is spelled out in rule 500.7:

500.7. Some effects can give a player extra turns. They do this by adding the turns directly after the current turn. If a player gets
  multiple extra turns or if multiple players get extra turns during a
  single turn, the extra turns are added one at a time. The most
  recently created turn will be taken first.

Let's look at how this works on its own: suppose you're playing a four-player game, players A, B, C, and D (in that order); the turn order would then normally be A B C D A B C D etc.  Now, suppose player A plays a Time Sifter.  At the start of player B's upkeep, the structure of the upcoming turns looks like (using [] to denote the current turn: [B] C D ...  Technically, the game doesn't actually know that C will be taking a turn after B; instead, it knows that the 'present' turn is controlled by B.  I'll denote this by starring that turn: [B*] C D A ...  Now, Time Sifter's effect resolves; we'll say that D wins.  An extra turn for D (we'll label extra turns with a ' symbol, so D') - is 'inserted' after the current turn: [B*] D' C D A ...  Now, B finishes their turn and it goes on to D's extra turn: B* [D'] C D A ...  Assume that now C 'wins' the Time Sifter fight.  Then the turn structure becomes B* [D'] C' C D A ...; D's extra turn finishes, the turn passes to C's extra turn, and the game all the while is remembering that B took the last 'non-extra' turn and that C is next in turn order: B* [C'] C D A ...  Things go on like this until Time Sifter is removed.
3) Now, the next piece of the puzzle: Radiate's interaction with Time Stretch.  I'll assume that during one of player A's 'extra' turns (so that the current structure is B* [A'] C D A ...) they cast Time Stretch (targeting themselves!), and that player B Radiates it.  As other answers have noted, when Radiate resolves copies of Time Stretch will be generated for players B, C, and D; these copies resolve before the original Time Stretch does.  I'll presume that player B stacks them with his copy on the bottom, D above, and C's copy on top, so that the resolution will be CDBA; this means that after all the copies and the original Stretch resolve, the 'turn stack' looks like: B* [A'] A' A' B' B' D' D' C' C' C D A ...
4) From here, Time Sifter will work as normal.  This means that (once A's turn ends) A will start to take the first of their extra turns, putting the Sifter on the stack: B* [A'] A' B' B' D' D' C' C' C D A ...  Now, suppose D wins.  Then a new extra turn for D will be added, but all of the other extra turns are still there: B* [A'] D' A' B' B' D' D' C' C' C D A ... and once the turn passes from A to D, it'll look like B* [D'] A' B' B' D' D' C' C' C D A ... - but all of those extra turns are still hanging out, waiting for Time Sifter to go away somehow so that they can be had.
I hope this helps somewhat - it's obviously a remarkably complex situation, but once you understand what each effect is actually doing it all flows relatively straightforwardly.

This is pretty much what I thought was supposed to happen. The visual representation with letters makes every aspect of this issue much easier to understand, and you addressed the multiple pitfalls this situation was presenting.

In 3, shouldn't the extra turns be ordered the other way around (i.e. starting with A's)? The original Time Strech resolves last and hence is the most recently created turn.

Ack, that's a very good catch- I was putting them up by resolve-order, but of course the extra turns stack is (essentially) a stack. Thank you!

First, the actual turn order is never changed. Rather, there's a stack of extra turns that must be taken before turn order can proceed.
Let's take a look at what happens. Presumably the player playing Time Stretch targeted himself. Let's call him "A".

Stack: Time Stretch[A]
Extra Turn Stack: -empty-

In response, you cast Radiate. It resolves. You get to place the copies in the order you desire.

Stack: Time Stretch[A], Time Stretch[you], Time Stretch[B], Time Stretch[C], Time Stretch[D]
Extra Turn Stack: -empty-

The spells start resolving.

Stack: -empty-
Extra Turn Stack: D,D,C,C,B,B,you,you,A,A

At the end of the current turn, the original caster will ge an extra turn.

Extra Turn Stack: D,D,C,C,B,B,you,you,A

During his upkeep, Time Sifter will give a player an extra turn. Just push that extra turn on the stack.

Extra Turn Stack: D,D,C,C,B,B,you,you,A,C

The Time Sifter keeps granting extra turns, so the stack doesn't get a chance to unwind. But what if the Time Sifter or its player were to leave the game? The extra turns have already been granted, so you don't lose them. Then game would continue to unwind the extra turn stack, giving "C" an extra turn, then giving "A" an extra turn, and so on until "D" has taken both of his extra turns.

Any turns that are "extra" turns get piled up at the end of the current one in a last-in first-out queue, much like the stack.  So if player 5 won this upkeep's's time sifter and player 2 cast the time stretch on themselves, your extra turn queue looks like this after it's all resolved
22-??-??-??-??-5
For the players in the middle, the person who cast the Radiate chooses the order the copies go on the stack, so they choose the order people take their turns in.  (Which will be the opposite of the order you stack them.)
Now the time sifter is going to jump back in and start creating extra turns at the front of the queue when player 2 takes their first extra turn, but that has no effect on this queue of extra turns sitting out there.  When the time sifter leaves play you'll finish taking all these extra turns that have been created and are waiting to happen, then resume regular turn order with the person who would have been next after the very first time sifter trigger.
I strongly recommend you keep a pen and paper handy to track this :)

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Can Scion of the Ur Dragon's ability fizzle a spell?
I want to know if Scion of The Ur-Dragon's activated ability can fizzle a kill spell or the like by becoming a copy of a card?
Does the ability make him another permanent as a response to any spell that would kill him essentially removing him as a valid target?

Not an answer, but worth mentioning since I've seen this sort of thing confuse a lot of people: Scion's ability isn't a mana ability. A mana ability is one that generates mana (with some restrictions), not one that costs mana.

@MichaelSnook Indeed, and I've edited the question to fix that issue.

@MichaelSnook Ah thanks for clearing that up. I can see why its misleading.

You can't reliably counter the spell.
Changing into a copy of something else doesn't make the Scion a different object. However, if the dragon it becomes is an illegal target (for example, due to having hexproof, or any other attributes that the target relies on) then that would work.

So if someone cast Murder for example, the target would still remain after scion changed?

@ThunderToes: That is precisely what it means. As Samthere stated, the permanent doesn't become a new object (which it would if it changed zones, for example).

@ThunderToes Yes, the object that was named Scion is still the same object, and still the target, even though the name and characteristics changed. Murder would still work. However, if the spell was Plummet, you could turn the Scion into Canopy Dragon and the spell would fizzle, since the target doesn't have flying.

Rightio, that clears this up then. Its kinda a good thing that its this way or Scion would be infinitely more OP than it already is haha.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Mirage Phalanx and Aurelia, the Warleader
If Mirage Phalanx soulbounds with Aurelia, the Warleader can it produce infinite combats? Does the legend rule remove the old copies of Aurelia in time for the soulbonding with the new copy to happen?

The original Aurelia is still paired with Mirage Phalanx when the copy enters play. This is because the "legend rule" that destroys the original Aurelia is a state-based action (specifically, rule 704.5j). Rule 704.3 describes the timing of state-based actions:

704.3. Whenever a player would get priority (see rule 117, “Timing and Priority”), the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event.

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

The copy effect is resolved and the copy enters play before the active player gets priority. But soulbond requires the creature to be unpaired at the moment the other creature enters play:

702.95a ... “Whenever another creature enters the battlefield under your control, if you control both that creature and this one and both are unpaired, you may pair that creature with this creature for as long as both remain creatures on the battlefield under your control.”

So the full sequence of events is:

Mirage Phalanx's ability starts resolving, putting a token copy of Aurelia into play.

Mirage Phalanx's triggered soulbond ability sees the token enter the battlefield, but since Mirage Phalanx is still paired with the original Aurelia, it does not trigger.

The ability finishes resolving.

The active player (you) would gain priority, so the game checks for state-based actions. It sees the two Aurelias and removes one by the legend rule.

I might be getting the terminology mixeds up, but isn't the point that Mirage Phalanx does not trigger when the token enters the battlefield because it is till paired with the original at that point?

@SwimmerF Exactly. First it checks to see if Mirage Phalanx is paired, and then it checks whether there are two Aurelias.

Only if you have Mirror Gallery or something similar in play.
Since Aurelia is a Legendary Creature, you'd normally only be able to have one in play at a time, thanks to the legend rule, so when Mriage Phalanx's ability triggers, you would then immediately choose to sacrifice either the original Aurelia or the token copy of her. However, cards that prevent this from occurring like Mirror Gallery exist; if you have one of these in play, then this wouldn't occur.
If the token copy of Aurelia persists, then you can potentially get infinite combat phases on your turn, since the Mirror Phalanx's ability triggers during the beginning of your combat phase, producing another Aurelia, and then one or both of the Aurelias attacks, triggering her ability and granting one (or two) additional combat phases and untapping all your creatures. Then, at the beginning of your next combat phase, Mirror Phalanx will trigger, creating another Aurelia, who can in turn create another combat phase when she attacks. This can repeat an indefinite number of times.

No, that does not work.
Here's the definition of soulbond, from the Comprehensive Rules:

702.95a Soulbond is a keyword that represents two triggered abilities. “Soulbond” means “When this creature enters the battlefield, if you control both this creature and another creature and both are unpaired, you may pair this creature with another unpaired creature you control for as long as both remain creatures on the battlefield under your control” and “Whenever another creature enters the battlefield under your control, if you control both that creature and this one and both are unpaired, you may pair that creature with this creature for as long as both remain creatures on the battlefield under your control.”

The second ability is the relevant one here. The requirement for both creatures to be unpaired is part of an “intervening ‘if’ clause”:

603.4. A triggered ability may read “When/Whenever/At [trigger event], if [condition], [effect].” When the trigger event occurs, the ability checks whether the stated condition is true. The ability triggers only if it is; otherwise it does nothing. If the ability triggers, it checks the stated condition again as it resolves. If the condition isn’t true at that time, the ability is removed from the stack and does nothing. Note that this mirrors the check for legal targets. This rule is referred to as the “intervening ‘if’ clause” rule. (The word “if” has only its normal English meaning anywhere else in the text of a card; this rule only applies to an “if” that immediately follows a trigger condition.)
Example: Felidar Sovereign reads, “At the beginning of your upkeep, if you have 40 or more life, you win the game.” Its controller’s life total is checked as that player’s upkeep begins. If that player has 39 or less life, the ability doesn’t trigger at all. If that player has 40 or more life, the ability triggers and goes on the stack. As the ability resolves, that player’s life total is checked again. If that player has 39 or less life at this time, the ability is removed from the stack and has no effect. If that player has 40 or more life at this time, the ability resolves and that player wins the game.

As that rule states, the condition is checked at the time the trigger event occurs (the creature entering the battlefield), while the copy-making triggered ability is resolving. At that time, Mirage Phalanx is still paired. The “legend rule”, being a state-based action, will be checked only after the copy-making triggered ability finishes resolving.

704.3. Whenever a player would get priority (see rule 117, “Timing and Priority”), the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event. If any state-based actions are performed as a result of a check, the check is repeated; otherwise all triggered abilities that are waiting to be put on the stack are put on the stack, then the check is repeated. Once no more state-based actions have been performed as the result of a check and no triggered abilities are waiting to be put on the stack, the appropriate player gets priority. This process also occurs during the cleanup step (see rule 514), except that if no state-based actions are performed as the result of the step’s first check and no triggered abilities are waiting to be put on the stack, then no player gets priority and the step ends.
704.4. Unlike triggered abilities, state-based actions pay no attention to what happens during the resolution of a spell or ability.
Example: A player controls a creature with the ability “This creature’s power and toughness are each equal to the number of cards in your hand” and casts a spell whose effect is “Discard your hand, then draw seven cards.” The creature will temporarily have toughness 0 in the middle of the spell’s resolution but will be back up to toughness 7 when the spell finishes resolving. Thus the creature will survive when state-based actions are checked. In contrast, an ability that triggers when the player has no cards in hand goes on the stack after the spell resolves, because its trigger event happened during resolution.

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


text:
Do cards with abilities that target trigger heroic?
Would something like Satyr Grovedancer or something that has "When ~ enters the battlefield, put a +1/+1 counter on target creature" trigger Heroic?
It has the keyword target, but does it count as an ability or as part of the spell that targets? I guess this goes likewise for any ability that occurs when a card enters the battlefield.

No, Heroic cards specifically state "whenever you cast a spell that targets [this card]". Abilities are not spells. Here is the definition of a spell:

111.1. A spell is a card on the stack. As the first step of being cast (see rule 601, “Casting Spells”), the card becomes a spell and is moved to the top of the stack from the zone it was in, which is usually its owner’s hand. (See rule 405, “Stack.”) A spell remains on the stack as a spell until it resolves (see rule 608, “Resolving Spells and Abilities”), is countered (see rule 701.5), or otherwise leaves the stack. For more information, see section 6, “Spells, Abilities, and Effects.”
111.1a A copy of a spell is also a spell, even if it has no card associated with it.

The Satyr Grovedancer's ability goes into the stack when it enters the battlefield which  doesn't happen until after it was cast (and subsequently resolved). The effect is on the stack, but is not spell because it is not a card being cast (or a copy of a spell).
Instants and sorceries that say "target" will trigger heroic. Enchantment auras will as well even though they do not say "target" because of rule regarding auras:

113.1b Aura spells are always targeted. These are
the only permanent spells with targets. An Aura’s
target is specified by its enchant keyword ability (see rule 702.5, “Enchant”). The target(s) are
chosen as the spell is cast; see rule 601.2c. An Aura permanent doesn’t target anything; only the
spell is targeted.

Anything else will be an ability and thus do nothing.

The important thing here is that when you cast a creature, the spell is just the creature. When it resolves, you put the creature onto the battlefield, and you're done with that spell. As it enters the battlefield, its ability triggers and goes on the stack, entirely separate from the creature spell.


text:
Can Eldrazi Processor effects "counter" madness?
In a couple of weeks cards with the madness mechanic, Eldrazi Processors, and Eldrazi Displacer are going to be in standard together. The text for Madness reads as follows:

“If a player would discard this card, that player discards it, but may exile it instead of putting it into his or her graveyard” and “When this card is exiled this way, its owner may cast it by paying [cost] rather than paying its mana cost. If that player doesn’t, he or she puts this card into his or her graveyard.”

With a board of Eldrazi Displacer and some ETB Processor like Wasteland Strangler, can I respond to a card being exiled through madness by displacing my Wasteland Strangler and putting the exiled card into the graveyard, thus effectively countering madness and triggering my process ability?
If this is the case what is to stop my opponent casting the exiled card in response to this?

If I were to ask this question again I would ask "Can Eldrazi Processors counter madness effects?", I didn't do my research properly as I now see that there are quite a few ways to process at instant speed without the help of Eldrazi Displacer.

You can always edit the question after the fact, as long as you don't change the core of what you're asking. In this case I've done it for you, but it's very much encouraged to do it yourself in the future. (Try not to edit too many times, though.)

Thank you for that, I wasn't sure what would happen if I edited something as major as the title. The mod edits people apply are useful as well as they teach me about accepted formatting on this page. i.e. hyperlinking linking card references.

Ah, since you mentioned hyperlinking: there is some disagreement about that. You can certainly learn from the way people edit, but keep in mind that it reflects the style of the individual editor, not necessarily the preferences of the community as a whole.

It's a pity SE doesn't support mouse overs on hyperlinks.

According to the SOI Mechanics Guide, madness will change in the new set so that discarding the card into exile is no longer optional.

Yes, you can remove a card discarded through Madness from exile before your opponent has a chance to cast it.
As per the the relevant rule you already quoted, Madness has two parts to it.

702.34a Madness is a keyword that represents two abilities. The first is a static ability that functions while the card with madness is in a player’s hand. The second is a triggered ability that functions when the first ability is applied. “Madness [cost]” means “If a player would discard this card, that player discards it, but exiles it instead of putting it into his or her graveyard” and “When this card is exiled this way, its owner may cast it by paying [cost] rather than paying its mana cost. If that player doesn’t, he or she puts this card into his or her graveyard.”

The first part says that the card with Madness is put into exile rather than the graveyard if it was discarded.
The second ability triggers off the card getting discarded to exile by Madness. Only if and when that second ability resolves can that player cast that spell for its alternative Madness cost. You can respond to the second ability by removing that card from exile with your Displacer/Processor combo. Since that causes the card to change zones and become a new object, the triggered Madness ability will lose track of that object and your opponent cannot cast that spell when the ability resolves.

Great answer thanks. Now to build an anti-madness ("You mad bro?") deck just in case grixis madness takes off.

Note that the rules will change with SOI release: The discard into exile is now mandatory, and choosing not to cast the card will put it into your graveyard. This has no effect on the rest of the answer, you can still 'process' the card before it can be cast. Source: http://magic.wizards.com/en/articles/archive/feature/shadows-over-innistrad-mechanics

That is interesting. From a flavor perspective it makes sense as madness shouldn't be optional. However mechanically it's slightly strange and was no doubt intended to reduce the synergy between madness and other mechanics in the set/JVP. Equally as enlightening is the section of that article entitled "No Flippy Floppy", I had been slightly concerned that we would be seeing 13 flip Avacyn kill combos.

Can I just confirm that all cards cast through madness can effectively be cast at instant speed anyway so there is nothing stopping someone casting a card exiled through madness in response to the activation of a processor ability?

Casting from madness can only be done as part of the resolution of the madness triggered ability. To process the card with madness, the process effect will have to be done in response to the trigger, so there is no opportunity to cast the card before it is processed. In addition, most instant speed "process" effects actually remove the card from exile as part of the cost, so that can't be responded to at all.


text:
Does River Heralds' Boon with one target count as one or two instances of adding +1/+1 counters?
River Heralds' Boon says:

Put a +1/+1 counter on target creature and a +1/+1 counter on up to one target Merfolk.

Winding Constrictor says:

If one or more counters would be put on an artifact or creature you control, that many plus one of each of those kinds of counters are put on that permanent instead.

If the same Merfolk is both the creature target and the Merfolk target, does Constrictor see two separate instances to replace, or one?

There's only one event in your scenario, so it would only get one extra +1/+1 counter.

River Heralds' Boon has only one verb, so there's only one instruction, so  it places the two counters simultaneously.
If you chose the same creature to receive both counters, there's only one event of "placing counters on an artifact or creature you control" since the counters are placed simultaneously, so three +1/+1 counters are placed instead.
If you chose different creatures to receive the counters, there are two events of "placing counters on an artifact or creature you control", so each creature will get two +1/+1 counters.
Finally, if there were multiple instructions to put a counter (as is the case for Lifecrafter's Gift and Common Bond), then each instruction would place an extra counter.

The same answer has been given (in less detail) by a judge in the Cranial Insertion article titled  "Don't Forget the Merfolk!"
Also of interest is the following ruling found on Winding Constrictor:

If an effect includes multiple instructions to put one or more counters on an artifact or creature, such as Lifecrafter’s Gift does, Winding Constrictor’s effect applies to each of those instructions.

Extra! Let's play "Count the events!"

700.1 [...] The text of triggered abilities and replacement effects defines the event they’re looking for. One “happening” may be treated as a single event by one ability and as multiple events by another.

Start by identifying the instructions because each instruction is performed atomically (all at once), and instructions are performed sequentially. This is done by identifying verbs.

"Do X. Do Y." has two instructions.
"Do X, then do Y." has two instructions.
"Do X, and do Y." has two instructions.
"Do X and Y." has one instruction.

Then you match the instruction's effect against the events defined by triggered abilities and replacement effects.
For the instruction "put two +1/+1 counters on target creature you control",

2 "placing a counter on a creature you control" events
1 "placing counters on a creature you control" event
1 "placing counters on creatures you control" event

For the instruction "put a +1/+1 counter on target creature you control and on a different target creature you control",

2 "placing a counter on a creature you control" events
2 "placing counters on a creature you control" events
1 "placing counters on creatures you control" event

For the instructions "put a +1/+1 counter on target creature you control, then put a +1/+1 counter on a target creature you control",

1 "placing a counter on a creature you control" event, then another.
1 "placing counters on a creature you control" event, then another.
1 "placing counters on creatures you control" event, then another.

I'm well on my way on a tutorial to identifying events there in the "Extra!", but that's all I have time for today.


text:
Changing targets for a spell with a variable number of targets
My opponent casts Paradoxical Outcome and targets 2 permanents they control.
I then cast Insidious Will. Now I can just counter Paradoxical Outcome, but instead I want to bounce some of their permanents (for example clearing blockers out of the way). It is clear that I can change the target from 2 permanents to another 2 permanents, but do I need to target the same number of permanents?
In other words can I choose to return none, one or seven permanents?
If I had to redirect the same number would I be able to choose the same target twice and as a result only return one?
Note that this is not a duplicate of "Choosing new targets for a spell with a variable number of targets" as that is specifically for copying spells, not changing the targets of an existing spell.

I am 95% sure that you can't change the number of targets, but I'm too lazy to look it up in the rules at this time.

Why would the copy make a difference? "You may choose new targets for target spell" vs "You may choose new targets for the copy." - either way you're choosing new targets for a spell.

@Jefromi The quotes in the answer on the linked question all specifically say copy e.g.: 706.10c Some effects copy a spell or ability and state that its controller may choose new targets for the copy. The player may leave any number of the targets unchanged, even if those targets would be illegal. If the player chooses to change some or all of the targets, the new targets must be legal. Once the player has decided what the copy’s targets will be, the copy is put onto the stack with those targets.

Okay, I guess the answer there quoted the more specific version of the rules - it's still just the general process of choosing new targets, just applied as part of copying a spell instead of to an existing spell. Sometimes the comprehensive rules have redundancies like that.

You cannot change the number of targets. The rules about targets say the following:

114.3. The same target can’t be chosen multiple times for any one instance of the word “target” on a spell or ability. If the spell or ability uses the word “target” in multiple places, the same object, player, or zone can be chosen once for each instance of the word “target” (as long as it fits the targeting criteria). This rule applies both when choosing targets for a spell or ability and when changing targets or choosing new targets for a spell or ability (see rule 114.6).
114.6. Some effects allow a player to change the target(s) of a spell or ability, and other effects allow a player to choose new targets for a spell or ability.

114.6a If an effect allows a player to “change the target(s)” of a spell or ability, each target can be changed only to another legal target. If a target can’t be changed to another legal target, the original target is unchanged, even if the original target is itself illegal by then. If all the targets aren’t changed to other legal targets, none of them are changed.
114.6d If an effect allows a player to “choose new targets” for a spell or ability, the player may leave any number of the targets unchanged, even if those targets would be illegal. If the player chooses to change some or all of the targets, the new targets must be legal and must not cause any unchanged targets to become illegal.

It might also be worth mentioning 114.3 to show that you can't choose the same target twice (e.g. by changing one target to another already-chosen target)

I've added that


text:
Do multiple instances of paying extra mana to target a permanent stack?
When I have two copies of Diffusion Sliver on the battlefield, does my opponent have to pay four mana, or two mana to target my slivers?
I mean it says, pay two, so when he pays two both cards should be happy, or does this stack?
An explanation how and why would be nice.

Related: Do multiple instances of Arcbond stack?

For convenience, I will quote Diffusion Sliver's ability:

Whenever a Sliver creature you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays {2}.

So, you've got two Diffusion Slivers. Let's call them Artemis and Boromir. Each of them has their own independent copy of this ability. When your opponent casts a spell that targets one of your slivers, you get two triggered abilities going on the stack: one from Artemis and one from Boromir.
When Artemis's triggered ability resolves, your opponent has to pay 2 mana to satisfy the cost of that ability (unless they let their spell be countered, but let's assume that doesn't happen). It's a general rule in Magic that you cannot pay for more than one cost using the same resources. So the two mana used to pay for Artemis's ability's cost cannot also pay for the cost of Boromir's ability. When they pay the mana, they no longer have that mana - just like money.
Later on when Boromir's triggered ability resolves, your opponent will have to find two additional mana to pay for that cost. That makes a total of four.

I might not have posted this answer only because I couldn't resist giving the slivers funny names.

@G.Kner Well, thanks. For what it's worth, I usually find it useful to wait a while - maybe a day or so - before accepting an answer, so that you can really choose the best one. You don't have to wait, of course, it's just something I've found useful in the past.

They have to pay four mana, when he casts the spell both slivers trigger and their ability to counter the spell unless he pays mana goes on the stack, to prevent these ability's from countering his spell they need to pay two mana for each of them making him pay a total of four mana

Is there a case where you pay the cost, like in this case, can you call it a cost?, for one of multiple triggered abilities, where the others are paid too?

Just to be sure, i always look at one spell after another on the stack, there is nothing that can influence the next spell with my actions from the previous? Except stuff like making the target invalid, or countering it?

This only happens when something triggers twice but the ability states it can only happen once a turn

Those are perfectly normal triggered abilities. Each ability triggers when a sliver is targeted with a spell or ability. The controller of the abilities gets to put them on the stack in any order he or she prefers and his opponent gets to choose to pay either 0, 2 or 4 colourless mana.
The fact that two cards have the same name does not change how triggered abilities work, or how they are put on the stack.


text:
Who gets enters-the-battlefield abilities with Bribery?
When I have Terastodon in my library and an opponent puts it into play under his control using Bribery, who gets to destroy the three noncreature permanents, him or me?

See also the rulings on Bribery: 'You put the creature onto the battlefield, so you control it and any “enters the battlefield” abilities it has.'

When your opponent puts your creature into play with Bribery, it acts just as if they played it themselves (or, perhaps, put it into play from their own library). It's their creature, and the fact that you happen to own the physical card itself doesn't matter, except for some effects that explicitly use the word "own" or "owner". Terastodon does not have any of those effects. So it's your opponent who gets to choose what permanents to target and whether to destroy them.
If Terastodon dies, though, it will come back to your graveyard, not your opponent's. Once a card leaves the battlefield, then it does matter who owns the actual card.

A very clear and comprehensive answer. Thank you for that !!

@Jacobel If you find an answer that satisfies your question, click on the check mark next to the answer to accept it. You can always come back and change the accepted answer if someone writes a better one.


text:
When I play Lord of Tresserhorn, can I sacrifice Doomed Dissenter and its Zombie token?
Let's say I have a Doomed Dissenter on the battlefield.

When Doomed Dissenter dies, create a 2/2 black Zombie creature token.

Then I play Lord of Tresserhorn.

When Lord of Tresserhorn enters the battlefield, you lose 2 life, you sacrifice two creatures, and target opponent draws two cards.

For the two creatures I must sacrifice, can I choose Doomed Dissenter, and then the Zombie token it creates? Do I need to choose two creatures that are on the battlefield before either one dies?

Does this answer your question? If I sacrifice a creature for a spell and it creates a token, can I sacrifice that token for the spell too?

@Soulis not an exact duplicate (this is effect, that is cost) but the parts important to the question are the same. Close enough to close.

No, you cannot.
The token is not created immediately; the triggered ability has to go on the stack and then resolve first. And it isn’t even placed on the stack until after you finish resolving the Tresserhorn enter-the-battlefield ability.

Handling Triggered Abilities

603.2. Whenever a game event or game state matches a triggered ability’s trigger event, that ability automatically triggers. The ability doesn’t do anything at this point.
603.3. Once an ability has triggered, its controller puts it on the stack as an object that’s not a card the next time a player would receive priority.

So the 2/2 black Zombie creature token is not created until well after both the creatures have been sacrificed for Lord of Tresserhorn.


text:
Does a Myr Welder token created with Mimic Vat retain abilities added to the original?
Suppose I play Myr Welder, then later I play Grindclock (or any artifact) and it gets countered or destroyed. Next turn I exile grindclock from my graveyard with Myr Welder's activated ability and play Mimic Vat. Myr Welder dies on the opponent's next turn and is exiled with Mimic Vat's imprint ability. Then on my next turn I activate the Vat and put a token copy of Myr Welder into play.
Does the token have Grindclock's abilities after Myr Welder had been exiled? Since the myr hit the graveyard first, I know it loses counters. But does the Grindclock remain "exiled with (Myr Welder)" allowing the use of its abilities?

This seems like something like it might have been asked before, although I can't find a really clear preexisting version. This one is close, as is this one.

@David Z, Both of those have 400.7 as an answer, but the direct reason the Grindclock isn't considered exiled by the Myr Welder token has nothing to do with zone changes. The OP could equivalently have asked about a Myr Welder clone created by Clone, for example, a scenario in which no zone changes occur.

@ikegami Yeah, I know, that's why I didn't vote to mark as a duplicate of either of those. I didn't find a question dealing with the relation between (token) copies of something and the original thing.

Well, you said the two linked answer were close, yet they don't mention 201.4 at all.

400.7 also applies here since myr welder hits the graveyard.

A hundred different rules "apply" in the scenario you presented. I didn't say 400.7 doesn't apply; I said it doesn't answer the question.

No.
When an object refers to itself by name, it refers specifically to that object (regardless of its name). It does not refer to other objects with the same name.
The Myr Welder card in exile never exiled anything.
The Myr Welder token on the battlefield never exiled anything.

201.4. Text that refers to the object it’s on by name means just that particular object and not any other objects with that name, regardless of any name changes caused by game effects.

400.7 also applies since myr welder must go to the graveyard first.

Sure, a hundred different rules "apply" in the scenario you presented, including 400.7. However, the answer to your question has nothing to do with 400.7.

Indeed. Just wanted to make sure that it it's clearly stated for any who comes looking


text:
When my opponent plays several spells at once, can I counter them all together?
My opponent has all these zero cost equipments and laid them all down at once and played some of his creatures spells without passing priority to me to see if I had a response. At the end of his whole thing I played a counter spell to counter all of it since he laid it all down at once. So was it all countered or not?

No, it doesn't work like that, not unless you're doing something very unusual (which I'm not even sure is actually possible).
For starters, your opponent can't actually cast multiple spells at once. When they put all their cards down on the table as you described, they're proposing a shortcut where they cast all the spells, one by one, and you decline to respond to any of them (i.e. you decline to counter them or whatever). You do not have to accept this shortcut. In other words, your opponent cannot deny you the opportunity to respond to their spells.1 If you do want to respond, you can tell your opponent something like "wait, back up, I have responses" and then they have to take the cards back into their hand and cast the spells one at a time, passing priority to you after each one1, and you tell them when they cast each of those spells whether or not you want to play something in response.
The other issue is that, even if your opponent did have multiple spells on the stack at once (as in the situation described in the footnote2), Counterspell and similar cards only counter a single spell. Just because there are a whole bunch of spells to choose from doesn't mean that you can actually apply Counterspell's effect to all of them at once. There are a few cards that can counter multiple spells, like Counterflux (when overloaded) or Summary Dismissal (which technically exiles rather than countering), but they can only do this because they say so on the card. Note the difference in wording compared to a standard Counterspell.

1 Except when split second is involved, but that's very exotic these days.
2 If you were asking about instants or cards with flash, then things would be a little but more complicated. Your opponent could cast a spell, then immediately cast an instant or flash card without passing priority, and then could cast another instant or flash card without passing priority, and so on for as many as they want to cast. But that only applies to instants and cards with flash, not to typical artifacts and creatures (which don't have flash), as it seems you're asking about here. And even in this case, none of these spells could resolve without you having a chance to respond to them first. Your opponent can never take that away from you.1

For accuracy, you can still respond to a Split Second spell, just not by casting spells or activating abilities. You could, however, unmorph a creature. Another point could be made that Split Second isn't any more exotic than it has been, seeing as there are only 15 cards with that keyword, and about a third of those are going very strong in Commander games.

Flusterstorm is another which can counter multiple spells at once.

In addition to unmorphing, you can activate mana abilities while split second is on the stack. This is usually pointless, but can be useful if the mana abilities cause some additional effect (such as sacrificing a creature).

Also Mindbreak Trap

What you're describing is (more or less) possible to happen, but it's more likely that your opponent intended to just cast everything one after another. In any case, keep in mind that you can ask your opponent to wait at any time if you need to consider your options, or clarify the game state.
Your opponent is actually casting multiple spells before passing priority
When a player casts a spell or an ability, that player gets priority next. This is known as holding priority (you may also want to read What are the uses of holding priority?)

116.3c If a player has priority when he or she casts a spell, activates an ability, or takes a special action, that player receives priority afterward.

This means that if your opponent casts a spell, they will have the opportunity to cast or do something in response to their own spell, before you're allowed to respond. Note that casting multiple Equipment spells this way requires them to be able to cast them at Instant speed, because they normally require the stack to be empty in order to be able to cast them.

116.1a A player may cast an instant spell any time he or she has priority. A player may cast a noninstant spell during his or her main phase any time he or she has priority and the stack is
empty.

Eventually, they will pass priority, with all of their spells on the stack. If you cast a Counterspell (which targets a single spell) now, just like choosing targets works on the battlefield, you may choose as many targets as the spell allows, except the targets here are spells on the stack. Each equipment will still be a single spell on the stack, meaning you can't counter more than one unless you have a card that's specifically made for this purpose (such as an overloaded Counterflux).
Your opponent is ignoring the rules of priority (in casual play)
This other scenario means that your opponent is simply not aware of all of the above, or actually not caring about being in a massive disadvantage with you being able to counter each of their spells. However, the nature of execution is relevant here, so let me cover some situations.
Your opponent is literally putting all cards into play at once
If they slam everything on the table at once, you need to find a way to sync up with the game rules again. You should ask them to determine a stack order for what they just did. Then you effectively can interrupt that order at a specific point, let's say countering the third spell they cast out of five. This means that you intend to let spell one and two resolve, counter spell three, with spell four and five not yet on the stack and your opponent not required to cast them afterwards.
Note that this is a very subjective interpretation of how a situation like this should be handled, which is based on the assumption that the end result should be as close as possible to legit ways of playing. You could always ask them to re-do everything they just did, going through the motions more thoroughly.
Your opponent is casting spells one after another
If your opponent is casting spells one by one, it's up to you to interrupt them and announce that you would like to counter their spell. Because of the way that holding priority works, the latest possible time you could do that is at the time you know that they wouldn't be able to cast an Equipment without Flash in response to another Equipment (which means you definitely know that they don't intend to hold priority).
Tournament Scenarios
In a tournament, players have to explicitly announce that they intend to hold priority; otherwise, it's assumed that they play spells and activate abilities one after another. This means that if your opponent does this in a tournament scenario, stop them right after the first card to either counter it, or get a moment to read the card and consider it. If you simply let the game progress, wait for your opponent to go through their motions, then say "well, I would have liked to do something way back" - that would be too late.
Of course, if there are any problems of this kind during a tournament match, don't attempt to solve this on your own, but immediately call a judge.

OP Sounds like a pretty new player - while what you're saying is right to the letter of the law it probably also doesn't mean much to them. In simple terms the opponent is playing a Cheerios deck so each equipment is going to be cast as normal, meaning each one needs to finish resolving before he can cast the next. Sadly for the OP he can only counter one at a time since the stack would never be larger than one equipment spell.

@LuckySevens: I see your point, but I would like you to consider the usefulness of the answer in its own right, not just to a presumably new player. I believe it's favorable to answer questions as exhaustive as necessary, and with this being a situation with a lot of outs, I went in quite some detail to cover all bases I could think of. I believe this to be helpful not just for the questioner, but also for everyone else interested in the question. I also think that they aren't as new as others seem to think, as they apparently have a good idea of what priority is, just not how to enforce it.

If the spells are equipment and creatures as the OP describes, they are (barring other cards) all being cast at sorcery speed, meaning they are necessarily being cast one after the other since putting them on the stack at once is not an option.

@Zags: Correct, and I also noted in my answer. Despite that, the question will still be found when searching for other cases, which is why I thought I'd cover all cases. It seems more beneficial to explain the concept behind it rather than the specific situation. The question is asked in a way that suggests that they are interested in a general explanation rather than "can you cast Equipment spells in response to each other" - I believe that "can I counter all spells on the stack with a single counter spell [and if not, how does it work]?" is the real question.

You and I, my friend, need to have a talk about the stack.
The stack is Magic's abstraction of time. At its core, it is what keeps everything from happening at once, while offering each player to react to each-other's actions.
To cite some rules

405.1. When a spell is cast, the physical card is put on the stack (see rule 601.2a). When an  ability is activated or triggers, it goes on top of the stack without any card associated with it  (see rules 602.2a and 603.3).
405.2. The stack keeps track of the order that spells and/or abilities were added to it. Each time an object is put on the stack, it’s put on top of all objects already there.

So we see that spells and abilities (triggered or activated) can be added to the stack. Now, to get them off of there again:

405.5. When all players pass in succession, the top (last-added) spell or ability on the stack resolves. [...]

Note that most spell-types have restrictions on when they can be played. For instance, sorceries:

307.1. A player who has priority may cast a sorcery card from his or her hand during a main phase of his or her turn when the stack is empty. [...]

And the same (with some variations) goes for lands, artifacts and equipment, enchantments, and creatures. The 'exceptions', those objects that can be added to the stack while it is not empty, are instants and abilities (triggered or activated) of permanents on the battlefield.

Now that the rulebook has been read, let's look at your game.

My opponent has all these zero cost equipments and laid them all down at once [...]

Technically, he cannot do this. Every one of those equipments needs to follow this pattern:

Stack is empty
He casts an equipment (announces the spell, pays the cost, places the card on the stack)
Note that at this time he cannot cast another equipment, as the stack is not empty anymore!

Now either one of these scenario's can happen:

he passes priority
you pass priority
The top of the stack resolves, an equipment enters the battlefield

Or:

he passes priority
you cast Counterspell
you pass priority
he passes priority and the top of the stack (your Conterspell) resolves
Counterspell removes the equipment from the stack and places it in your opponents graveyard. Counterspell itself is removed from the stack and placed in your graveyard.

Or, even more complex:

he passes priority
you cast Counterspell
you pass priority
he counters Counterspell with Manaleak
you both pass priority
mana Leak removes Counterspell
Now you both get to pass priority again before the new top-of-the-stack (the original equipment) resolves
you both pass priority
The top of the stack resolves, an equipment enters the battlefield

And whenever the last item is removed from the stack, leaving it empty, someone can add a spell to it again and start building a new stack.

Note that there are a lot more complexities when looking at the stack - half the fun in Magic is putting stuff on the stack in the right order - , but this should give you a working understanding to get started with.

"The stack is Magic's abstraction of time." - I would argue that this isn't the best choice of words, for once because it introduces that MtG has a concept of time (which I don't believe it has), second because there are other elements that could be interpreted as abstractions of time, such as the turn/phase/step model, and finally because there are other things in the game (such as timestamp order) that care about "time", but not the stack. The stack is first and foremost simply a game zone.

@TheThirdMan "Time is what prevents everything from happening at once." - John Archibald Wheeler. I choose this wording specifically to show that the stack helps to visualise in what order (ie when in time) things happen.

The stack is in no way meant to help you visualize in what order things happen in a game of Magic in general, and even if you'd consider the stack on its own (which I guess is what you mean, but never state), sequence is different enough from the concept of time in our physical world that the phrasing allows for more false conclusions than it helps understanding (especially if you consider that things always resolve in opposite order). I further think that looking at time and any arbitrary sequence as being the same difference isn't something a lot of physicists would sign.

I'd say you're both in the wrong. Counterspell counters 1 spell as it's being cast, so you can't just counter everything in one go like that. On the other hand, if you're playing properly then he should have cast one spell, given you the chance to counter it, then let it resolve and cast the next one and repeat. Otherwise, you can just work out which of the spells it's most important for you to counter, and say that you're countering that one - and passing priority properly would mean you don't get that opportunity.


text:
Do I have to spend 4 mana on Nissa, Steward of Elements to put 1 counter on her?
Do I have to cast Nissa, Steward of Elements for 4 mana in order to put 1 loyalty counter on her?

If Nissa is cast for 4 mana, you are declaring X = 2, so she would enter the battlefield at 2 loyalty.
The minimum you can cast her and have her not immediately be put into the graveyard is 3 (X = 1).

No, only three mana ({1}{G}{U}) is required for that.
First of all, note that X is not derived from the cost, the cost is derived from the X you choose. (This matters if there are cost increases or cost reductions in play.)
If you choose X=0, Nissa costs {G}{U} to cast[1], and she enters the battlefield without any loyalty counters (and dies immediately after).
If you choose X=1, Nissa costs {1}{G}{U} to cast[1], and she enters the battlefield with one loyalty counter.
If you choose X=2, Nissa costs {2}{G}{U} to cast[1], and she enters the battlefield with two loyalty counters. This is the highest X you can choose if you have four mana.

Subject to cost increases and reductions, as usual.

Oath of Gideon as a caveat to X=0 causing Nissa to die.


text:
How is the CMC of a spell obtained for Vial Smasher?
Suppose I'm running a Vial Smasher the Fierce deck and I cast Blazing Shoal with X = 5. I've read in other answers to different problems that the converted mana cost of Blazing Shoal would be 7 while it is on the stack. I assume that then Vial Smasher's ability would trigger. Looking at Blazing Shoal, would the CMC be 7 or 2?

I'm not clear what the question is here. You read that the converted mana cost of Blazing Shoal would be 7 while it's on the stack. Do you have some reason to doubt that? If so, that would be important information to include in the question.

Ruling

11/8/2016    For spells with Variable Colorless in their mana costs, use
  the value chosen for X to determine the spell’s converted mana
  cost.

The Comprehensive Rules

202.3d 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.

So..X=5, making CMC 7.

107.3f If a card in any zone other than the stack has an {X} in its mana cost, the value of {X} is treated as 0, even if the value of X is
  defined somewhere within its text.

Clarification.
In Magic there are no in-between zones. When you cast a spell it goes from your hand (hand zone) to the stack (stack zone). As long as it's in your hand it is not considered cast. When it leaves the zone of your hand it will at the same time enter the zone of the stack making it a spell that at that point will be considered cast.
So whenever you cast anything that has an {X} in it, the X will become the amount you paid for it and move to the stack zone where {X} it will retain its value until it leaves the stack zone.

You cast the spell onto the stack. When the spell is on the stack x=5so the CMC would be 7 when Vial Smasher the Fierce ability triggers.


text:
Is exchanging life the same as losing life?
So say I'm playing a Rakdos, Lord of Riots deck and I tap Tree of Perdition exchanging a life total of 40 from one of my opponents with Tree of Perdition's toughness of 13. Do the opponents count as having lost 27 life, or was it different because it was just exchanged? Could I cast a 27/27 Endless One, just to name one example, or was there no actual loss of life?

The relevant rules governing exchanges of life totals are these:

701.9c. When life totals are exchanged, each player gains or loses the amount of life necessary to equal the other player's previous life total. Replacement effects may modify these gains and losses, and triggered abilities may trigger on them. A player who can't gain life can't be given a higher life total this way, and a player who can't lose life can't be given a lower life total this way (see rules 118.7-8).
701.9g. A spell or ability may instruct a player to exchange two numerical values. In such an exchange, each value becomes equal to the previous value of the other. If either of those values is a life total, the affected player gains or loses the amount of life necessary to equal the other value. Replacement effects may modify this gain or loss, and triggered abilities may trigger on it. A player who can't gain life can't be given a higher life total this way, and a player who can't lose life can't be given a lower life total this way (see rules 118.7-8). If either of those values is a power or toughness, a continuous effect is created setting that power or toughness to the other value (see rule 613.3b). This rule does not apply to spells and abilities that switch a creature's power and toughness.

In both cases, an exchange does cause a gain or loss of life. So yes, in the situation you describe, your opponent has lost 27 life and Rakdos's abilities will recognize that.

Excellent point by @David Z, but also just to point out the cards ruling for absolute clarity

13/07/2016
When the ability resolves, Tree of Perdition’s toughness becomes the
  targeted opponent’s former life total and that player gains or loses
  an amount of life necessary so that his or her life total equals Tree
  of Perdition’s former toughness. Other effects that interact with life
  gain or life loss will interact with this effect accordingly.

Paying special attention to

Other effects that interact with life loss will interact with this affect accordingly

So your opponent has lost 27 life and Rakdos's abilities will trigger on that basis.


text:
Does Skyward Eye Prophets get an extra use of its ability because it has vigilance?
So I was playing some commander with a friend and lent him my Derevi, Empyrial Tactician deck, which had Skyward Eye Prophets in it which basically taps to do a Coiling Oracle. He claimed that somehow because Skyward Eye Prophets has vigilance, he can cheat out an extra use with it while attacking, and I don't really understand how this works. Could someone please explain it?
I'm not sure if i'm just misunderstanding the vigilance keyword ability in general, but my understanding is the creature just doesn't tap when it attacks. I don't really think what he was doing works but he's much more experienced than me so I'm not really sure.

When you say "cheat out an extra use", what exactly do you mean?

He claimed that he could tap it while attacking but somehow it would untap because it had vigilance and then after combat he could tap it again for a "regular" use. He was not using Derevi's ability to untap permanents either.

I fixed up the formatting of your question a little bit, Arowaaron, but I'd encourage you to also add the information from your comment into the question itself.

Your friend is either playing incorrectly or explaining things poorly. Something sounds seriously muddled up. I'm going to explain how things could legally happen here, then explain the mistakes that could be getting made.
Also, you're right: vigilance simply means "attacking doesn't cause this creature to tap." It doesn't do any untapping, it simply makes declaring an attacker not tap it in the first place.
The Combat Phase
As a quick reminder, let's go through how the combat phase works:

Beginning of Combat Step. "At the beginning of combat" abilities trigger, then players have a chance to cast spells and activate abilities.
Declare Attackers Step.

The attacking player chooses any number of creatures they control that they wish to declare as attackers. Those creatures must be untapped at this point — you can't attack with a tapped creature. They choose which player or planeswalker those creatures are attacking, then tap all their declared attackers except the ones with vigilance.
Any triggered abilities just triggered go on the stack (e.g. Territorial Hammerskull) then players can cast spells and activate abilities.

Declare Blockers Step.

The defending player(s) choose any number of untapped creatures to declare as blockers (including none), then assign them legally as blockers to specific attackers.
Any triggered abilities just triggered go on the stack (e.g. Pride Guardian) then players can cast spells and activate abilities.

Combat Damage Step. Players say how damage will be assigned, then all of it is assigned simultaneously. Then we do triggered abilities again (e.g. Daring Saboteur) and players can cast spells and activate abilities again.
End of Combat Step. One last chance to cast spells and activate abilities before it stops being combat and creatures stop counting as attacking/blocking (e.g. Divine Verdict).

How this can legally happen
Let's call your friend Sam, and Skyward Eye Prophets just "SEP".

Sam has Derevi out on the battlefield already.
Declare attackers step: Sam attacks with SEP.
Declare blockers step: The defender cannot or does not block SEP.
During that step, Sam knows they're in the clear. Sam taps SEP for its ability and resolves it.
Combat damage step:

SEP deals combat damage.
Derevi's triggered ability lets Sam choose to untap SEP. Sam does so.
Sam uses SEP's ability again, tapping it.

You can see none of that involves "cheating out an extra use while attacking" exactly, and none of it has much to do with Vigilance (other than Vigilance didn't make Sam tap SEP as an attacker). Sam just entirely normally gets an extra use out of dealing combat damage if they choose to untap SEP.
In fact, Sam could get lots of uses out of SEP at that stage: for each creature of theirs that deals Combat Damage whilst Derevi is out, they can choose to untap SEP, and activate its ability each time. (Understanding how that works would require a good understanding of the stack & priority. If you're not familiar with those concepts, I recommend reading this answer, which helped me understand them when I was new.)
But none of those are "cheating out an extra use while attacking".
The mistakes that could be being made here
Your friend could be doing things 100% by the rules, just explaining things poorly.
Your friend could also be mistaken about how the rules work. If so, you and/or your friend could be thinking a combination of any/all of these.

✘ Incorrect: Vigilance causes a creature to untap.

✔ Correct: Vigilance just causes it to not tap as attackers are declared, as we covered earlier.

✘ Incorrect: Tapped creatures can be declared as attackers (which then untaps them if they have Vigilance).

✔ Correct: Only untapped creatures can be chosen as attackers during the Declare Attackers step.

✘ Incorrect: If you attack with a creature (and tap it), all of its abilities with a {T} (tap symbol) in the cost are also activated simultaneously.

✔ Correct: Doesn't happen. You usually have to choose between "tap for that ability" or "tap as an attacker" during your turn. Part of the beauty of a creature with Vigilance and a tap ability (such as SEP, or Vona, Butcher of Magan) is you can attack with it and then also use its {T} ability.
Some creatures (like Frenzied Goblin) have a triggered ability of the format: "When (this creature) attacks, you may pay (cost). If you do, (stuff happens)." These you can use as a part of attacking, because it's just a triggered ability (see step 2.2 above) and you can pay that cost to have the stuff happen.

✘ Incorrect: You can pay two costs with the same payment. If I'm tapping a creature to attack, I can also use that tapping to pay a cost of {T}! Also, I can activate the abilities of both Fathom Fleet Firebrand and Frontline Devastator with the same 2 mana: I am paying 2 mana, therefore I activate all abilities that involve paying 2 mana.

✔ Correct: You cannot pay two costs with the same payment. You choose one thing to do, then pay the cost for that one thing. Then you may choose other things to do, and pay those costs. But all the costs are paid separately to each thing. (Much like you cannot buy a $5 lunch from ten different shops with the same $5 bill.)

✘ Incorrect: Damage happens instantly as soon as I declare an attacker.

✔ Correct: There's a Combat Phase process to follow, and attackers don't deal their damage until near the end of that process.

In general, also bear in mind M:tG is a game with strict steps we move through one after the other: turn structure occurs in specific steps, so do the concepts of the stack and priority. We can't go wibbly-wobbly with the steps without breaking the rules, and those steps keep things pretty orderly.
When in doubt about something complex somebody's doing, ask them to explain about the sequence of events that allow them to do it, and make sure it makes sense to you at each step and that it isn't breaking any game rules. There's been several times someone's tried to do something that didn't seem right, and through asking for that explanation, I either learned how it worked, or we spotted that some part of the sequence meant they couldn't actually do that thing they were trying to do. (Not out of cheating, mind — this is a complex game sometimes, and when it does get complex we can all do with a hand keeping things working right.)


text:
What happens in multiplayer Magic when I'm forced to attack and one opponent has Propaganda?
While playing a multiplayer game of commander I stumbled upon an interesting predicament. One player has a Thantis, the Warweaver out and I am forced to attack. There are three other players and one has a Propaganda so I can't attack them unless I pay {2}.
When I go to combat can I choose to target the player with Propaganda and not pay the {2} therefore not attacking, or am I forced to choose another player that I can attack and swing at them?
The rulings I glanced over said if I have to swing at someone with a Propaganda or the like, I may choose not to pay the costs and not attack, but I never saw anything dictating what to do if there are more players who I can attack.

Attacking doesn't target. Attacking doesn't use the stack. There's no such thing as choosing to attack, and then not attacking. A creature either attacks, or it doesn't. What I'm saying is your creature can't attack the player with Propaganda without paying the cost.
You have these options:

Your creature can attack the player with Propaganda (which requires paying {2}), or
your creature can attack a player without Propaganda.
your creature can attack a planeswalker.

As you've said, you are free to choose not to pay the cost imposed by Propaganda[117.3c, 508.1d]. But if you declare to have your creature attack the player with Propaganda, and if you choose not to pay the cost imposed by Propaganda, the attack becomes illegal[508.1c]. Since you can't choose to take an illegal action, the action is rolled back[508.1, 721.1], and you must choose a different player or planeswalker to attack[508.1d].
Only if there are no valid choices (e.g. every opponent has a Propaganda, no opponent has a planeswalker, and you refuse to pay the cost imposed by Propaganda) can you avoid attacking[508.1d]. This is an application of second Golden Rule of Magic, popularly paraphrased as «"can't" trumps "can"»[101.2]

117.3c Activating mana abilities is not mandatory, even if paying a cost is.
508.1. [...] If at any point during the declaration of attackers, the active player is unable to comply with any of the steps listed below, the declaration is illegal; the game returns to the moment before the declaration [...]
508.1c The active player checks each creature they control to see whether it’s affected by any restrictions (effects that say a creature can’t attack, or that it can’t attack unless some condition is met). If any restrictions are being disobeyed, the declaration of attackers is illegal.
508.1d The active player checks each creature they control to see whether it’s affected by any requirements (effects that say a creature attacks if able, or that it attacks if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of attackers is illegal. If a creature can’t attack unless a player pays a cost, that player is not required to pay that cost, even if attacking with that creature would increase the number of requirements being obeyed. If a requirement that says a creature attacks if able during a certain turn refers to a turn with multiple combat phases, the creature attacks if able during each declare attackers step in that turn.
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.
721.1. If a player takes an illegal action or starts to take an action but can’t legally complete it, the entire action is reversed and any payments already made are canceled. [...]

Thanks for the answer! But the question is only one person has a Propaganda with a few other options to attack. Must I attack the other players if I dont pay the Propaganda for the one player, or did I find a new way to get past forced attackers?

ok, so in the end i have to attack someone else unless the all have Propaganda. There was a lot there and only a small part that kinda said it, sorry.

from this answer (508.1d) and the gatherer rules it seems you can choose to attack the player with propaganda and then choose not to pay the cost. i think you two misunderstood each other there

@user23545, You are free not to pay for Propaganda, but if you don't, the attack is illegal. (508.1c The active player checks each creature they control to see whether it’s affected by any restrictions (effects that say a creature can’t attack, or that it can’t attack unless some condition is met). If any restrictions are being disobeyed, the declaration of attackers is illegal.) You must therefore make a different choice of player or planeswalker to attack if able. I'll add this to my answer when I'm at my computer

@user23545, I have done so

'and you "must" choose a different player or planeswalker to attack'  You Must, or you May?

@Ricky Mason, Must. Remember, "All creatures attack each combat if able" is part of the scenario. If you can attack and don't, you would be satisfying 0 requirements when you could satisfy more than 0, violating 508.1d. Initially making an illegal choice doesn't absolve you of the need to attack. If so, you could say "I attack the moon" to get around "All creatures attack each combat if able."

@ikegami, it is much clearer now

You must attack. However, I'll take a probably unpopular stance and say that the rule supporting this conclusion is slightly ambiguous.
If you attack, then you are obeying 1 requirement. If you do not attack, then you are obeying 0 requirements. Therefore, per rule 508.1d, you must attack.

508.1d [...] If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of attackers is illegal. [...]

To declare attackers, the player follows rules 508.1a-m in order. When the player reaches rule 508.1d, they must calculate the "maximum possible number of requirements that could be obeyed without disobeying any restrictions". "Could" in this case is ambiguous. What the rule actually means is:

What is the maximum possible number of requirements that could be obeyed without disobeying any restrictions, if the game is rolled back to step 508.1a?

That's just how the rule is meant to be interpreted.

Did you mean "without rolling back the game to step 508.1a" instead of "if the game is rolled back to step 508.1a?"? Or, put more clearly, did you mean "What is the maximum possible number of requirements that could be obeyed legally without disobeying any restrictions?"

And if it's ambiguous without adding "legally", what would be the other interpretation?

According to the metaphore, the rules check if the creature attacked. In reality, they check if its attacking ("requirements are being met" rather than "requires were met"). The rules treat attacking as an ongoing action, not a instantenous effect like eating a cookie in your metaphore. That avoids the ambiguity you claim exists.

Yeah, already deleted.

Could you give an example of where it would make a difference? And if you're going to claim the rules are wrong, could you provide more backing?

Anyway, the question wasn't about whether they were required to attack or not, the question was whether attacking the Propaganda-protected played and not paying the cost counts as attacking. That leaves an answer that doesn't answer, and a vague claim that the rules are wrong.

Calculating the max number in 501.1a instead of 508.1d


text:
How are the replacement effects of lands such as Breeding Pool applied?
I am not sure how many replacement effects the rules text of lands such as Breeding Pool apply, and what options are available for applying them.
Suppose the active player controls an untapped Archelos, Lagoon Mystic and attempts to put a Breeding Pool into play.  Which of the following accurately describe the situation?

Breeding Pool has one replacement effect, roughly stated, "Replace Breeding Pool entering the battlefield with either the player paying 2 life and Breeding Pool entering the battlefield, or Breeding Pool entering the battlefield tapped."  Thus there are a total of two replacement effects to be applied to this event.  The player chooses which replacement effect to apply first and which to apply second, and whenever they apply Breeding Pool's replacement effect, they also choose which of its two options to apply.

Breeding Pool has two replacement effects, but only one is immediately available.  Breeding Pool's replacement effect to enter tapped is "unlocked" if and when the player applies its life-paying replacement effect and chooses not to pay.  Thus there are a total of two or three replacement effects to be applied to this event, depending on the choice made for Breeding Pool's life-payment replacement effect.  One legal option would be for the player to first apply Breeding Pool's life-paying replacement effect, choosing not to pay, then apply Archelos's replacement effect, and finally apply Breeding Pool's replacement effect to enter tapped.

Breeding Pool has two replacement effects, both immediately available.  Breeding Pool's replacement effect to enter tapped, if it has not yet been applied, "disappears" if and when the player applies its life-payment replacement effect and chooses to pay.  It's life-payment replacement effect, if it has not yet been applied, "disappears" if and when the player applies its replacement effect to enter tapped.  Thus there are a total of two or three replacement effects to be applied to this event, depending on the choices made.  The legal options for the player here seem similar to those in #2, but I believe they may differ in more complicated situations involving multiple players' replacement effects.

It is not clear to me what practically different outcomes you are suggesting could happen with the different options you mention.

@murgatroid99 I consider my question to be agnostic as to whether the set of attainable outcomes differs in each model.  More broadly, games have just three outcomes, win/lose/draw, but players must engage with the choices they're given to implement their desired outcomes, so I think an accurate mental representation of how these replacement effects are structured is valuable, even if the underlying choices are sometimes lost in outcome-oriented shortcutting.  In that respect, at least #1 certainly practically differs from the others.  Also, one relevant assumption I made uncritically when I

asked the question is that the choice of whether to pay 2 life is made at the time Breeding Pool's replacement effect(s) is chosen to modify the ETB event, within the replacement/prevention effect application algorithm, as opposed to at the time of the ETB event, after all other replacement effects have been applied.  I think this distinction is also relevant to whether the different models produce different sets of attainable outcomes.

I did a bit of searching and found a few sources confirming that Breeding Pool's ability does create a replacement effect; the most authoritative is probably this post on the Magic judges' rules blog. It doesn't explicitly say that there is exactly one replacement effect rather than two, but it's hard to read that post in a way that suggests there could be two replacement effects. (Plus, given my own experience with Magic, I would never think that Breeding Pool's ability would create two replacement effects.) So I'm fairly confident stating that Breeding Pool has one replacement effect modifying how it enters the battlefield, and you just have to make a choice to determine what that replacement effect actually does. Therefore, your options 2 and 3 are incorrect.
Then, there's a ruling on Archelos, which you can view on Gatherer, that says

If another replacement effect says that a permanent enters the battlefield tapped while Archelos is untapped, the entering permanent's controller chooses whether the permanent enters tapped or untapped. If a permanent is simply put onto the battlefield tapped without a replacement effect being applied, it always enters untapped if Archelos is untapped.

Even though it doesn't explicitly say so, I believe that the reason the permanent's controller gets to choose whether it enters tapped or untapped is precisely because they get to choose the order in which to apply replacement effects. That implies that your option 1 is correct.
Depending on the player's choices, these are the various ways this could play out, if Archelos is untapped:

Apply Breeding Pool's own replacement effect first, then Archelos's effect

Choose to pay 2 life
In this case the event of "Breeding Pool enters the battlefield" (untapped, implicitly) is modified by Breeding Pool's own effect to "Breeding Pool enters the battlefield and you pay 2 life", then further modified by Archelos's effect to "Breeding Pool enters the battlefield untapped and you pay 2 life"

Choose not to pay 2 life
In this case the event is modified by Breeding Pool's effect to "Breeding Pool enters the battlefield tapped" and then further modified by Archelos's effect to "Breeding Pool enters the battlefield untapped"

Apply Archelos's replacement effect first, then Breeding Pool's replacement effect

Choose to pay 2 life
In this case the event is modified by Archelos's effect to "Breeding Pool enters the battlefield untapped" and then further modified by Breeding Pool's effect to "Breeding Pool enters the battlefield untapped and you pay 2 life"

Choose not to pay 2 life
In this case the event is modified by Archelos's effect to "Breeding Pool enters the battlefield untapped" and then further modified by Breeding Pool's effect to "Breeding Pool enters the battlefield tapped"

Obviously, one of these is distinctly better for you than the others. :-)

I don't think you need to go looking for "sources" to confirm that Breeding Pool's ability creates a replacement effect. It's just one of the categories of replacement effect listed in rule 614.1d.

I thought it was important to back that up since it's at the core of the question. It's just that the source I happened to find most quickly was on the rules blog rather than the comprehensive rules. (anyway I would delete this answer to avoid confusion but I can't since it's been accepted)

Conversely, if Archelos is tapped, you can pay the 2 life to have Breeding Pool enter the battlefield untapped, right?

Breeding Pool has one replacement effect.
Breeding Pool's ability says

As Breeding Pool enters the battlefield, you may pay 2 life. If you don't, it enters the battlefield tapped.

The part that says "...you may pay 2 life. If you don't..." is a cost, as defined in rule 118.12:

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

When you are applying Breeding Pool's replacement effect, you choose whether to pay the cost of 2 life. If you do, nothing happens, and if you don't, the land enters the battlefield tapped.