MTG Salvation

In general, you may insert an opaque sleeve on a double-faced card so that only the back face of that card shows. Nothing in the comprehensive rules or the Magic: The Gathering Tournament Rules disallows this (for sanctioned tournaments, review M.T.R. 3.10).

Remember, though, that inserting a sleeve on a double-faced card to show the back face rather than the front face doesn't change how that card is seen by the game, and that any player that can look at a double-faced card can look at both its faces (C.R. 712.2). For example, a double-faced card in your library or your hand has only the front face's characteristics (C.R. 712.4a), even if—

• you inserted an opaque sleeve on that card to show only its back face, or
• you don't intend to play or cast the front face of that card (in the case of a modal double-faced card).

See also:

• https://www.reddit.com/r/mtgrules/comments/oh7g6d/dark_confidant_and_modal_doublefaced_kaldheim/
• https://www.mtgsalvation.com/forums/magic-fundamentals/magic-rulings/825476-pathway-lands-in-edh-question

Copy Enchantment says "You may have Copy Enchantment enter the battlefield as a copy of any enchantment on the battlefield" (C.R. 108.1).

Clever Impersonator says "You may have Clever Impersonator enter the battlefield as a copy of any nonland permanent on the battlefield" (C.R. 108.1).

Thus these abilities do exactly the same thing, except the set of permanents you can choose from is different in general.

However, it's unclear under the comprehensive rules whether a bestowed Aura permanent's effect of it becoming an Aura permanent is a copiable effect. Indeed:

• Under C.R. 702.103c: "If a bestowed Aura spell is copied, the copy is also a bestowed Aura spell. Any rule that refers to a spell cast bestowed applies to the copy as well." But this doesn't apply to bestowed Aura permanents.
• Under C.R. 702.103b, a spell cast bestowed "becomes an Aura enchantment and gains enchant creature", but it's unclear whether this effect is a copiable effect, especially because it behaves similar to mutating creature spells (which do produce a copiable effect) when the target becomes illegal (C.R. 613.2a, 608.3b).

Ravenous Chupacabra's ability can't target a permanent with protection from creatures (such as Serra's Emissary) if Ravenous Chupacabra is a creature (C.R. 702.16b).

Neither can Fury's enters-the-battlefield ability target a permanent with protection from creatures if Fury is a creature (C.R. 702.16b). (And if Fury would deal damage to a creature with protection from a quality shared by Fury, that damage is prevented [C.R. 702.16e].)

Note that both triggered abilities specify what they target by using the word "target" (C.R. 115.1d).

EDIT (Jul. 31): Correctness edit.

A token created with Hofri Ghostforge will have the copied creature's copiable power and toughness, and not necessarily be 1/1 (and due to Hofri Ghostforge, the token will also get +1/+1 and have haste if the token is a "Spiri[t] you control").

The effect of Storm of Souls making a returned creature "a 1/1 Spirit with flying in addition to its other types" is not a copiable effect (rather it's an effect that changes types, abilities, power, and toughness), so that effect won't carry over to an object that's a copy of that creature (C.R. 613.1d, 613.1g, 700.2; compare with C.R. 613.1a).

EDIT (Jul. 31): Correctness edit.

The Meathook Massacre's first ability generates an effect that applies only to creatures that are on the battlefield as the ability resolves, and not to creatures that enter the battlefield later in the turn. This is because it's an effect of a resolving triggered ability that changes objects' characteristics (in this case, power and toughness [C.R. 109.3]) (C.R. 611.2c).

If you control both Alms Collector and Spirit of the Labyrinth and an opponent of yours would draw two cards—

• as a cost or as a choice (e.g., as found in Sylvan Library or Consecrated Sphinx), that player can't pay that cost or make that choice (C.R. 121.2b), so Alms Collector's effect can't replace that with you and that player each drawing one card instead. In general, if something is impossible, it can't be replaced (C.R. 614.7, 101.2, 101.3; see also C.R. 614.17c).
• other than as a cost or choice (e.g., as found in Divination), and that opponent did not draw any cards yet this turn, the "(i)nstructio(n) to draw [two] cards [is] still ... partially carried out" (C.R. 121.2b, 101.3), so that the player draws only one card and Alms Collector's effect doesn't apply anymore.
• other than as a cost or choice, and that opponent did draw one or more cards this turn, that player simply doesn't draw any cards, so that Alms Collector's effect doesn't apply anymore (C.R. 101.2).

See also this thread.

EDIT (Oct. 10, 2020): Correction. Some rules were renumbered in the meantime.
EDIT (Jul. 2, 2022): Update rule renumberings.

The comprehensive rules arguably don't clarify enough whether the text "you would create one or more Treasure tokens" on Xorn is intended to mean "you would create one or more tokens that are Treasures". None of the rulings for Xorn, Academy Manufactor, or Jolene, the Plunder Queen (currently the only cards with effects that care whether "you would create ... token[s]" of a particular kind) clarifies this matter. (See also Vazi, Keen Negotiator.)

Note that effects that say to create a "Treasure token" create a "colorless Treasure artifact token with" a particular activated ability, by default (C.R. 111.10, 111.10a). Thus, the name of such a token is "Treasure Token" by default (C.R. 111.4). Although C.R. 111.10a defines what a "Treasure token" is for this purpose, it arguably doesn't define what a "Treasure token" is for other purposes, including for purposes of Xorn's "If you would create one or more Treasure tokens".

See, for example, Intangible Virtue, which applies to "[c]reature tokens you control".

EDIT (Jul. 1): Edited to add "currently".

If the card returning to the battlefield is a modal double-faced card, it must enter with its front face up (C.R. 712.10).

Remember that in general, you can choose between a modal double-faced card's front or back face only at the moment you're playing or casting that card (C.R. 712.7, 712.8), and not when you're moving it from one zone to another for any other reason.

Quote from Hammerer »

Sure I understand that copies are already attacking and they are not enchanted so they don't get Double Strike from Reyav ability, but Myriad copy abilities that are until end of turn, right? So I attack with Hammers of Moradin, they gain Double Strike until end of turn so Myriad copies are copied with Double Strike because original Hammers of Moradin got it or am I wrong?

That is wrong.

Effects of the form "[something] gains [ability] until end of turn" are not copiable effects, so they won't carry over to copies. For example, if a creature has "double strike until end of turn" due to Reyav's ability, and another permanent (such as a token [C.R. 111.6]) becomes a copy of that creature, the latter permanent will not necessarily have double strike.

In effect, each time Reyav's ability resolves it applies an effect to a set of creatures to grant double strike to (namely, the set consists of the creature that attacked), and after that set is determined it won't change (C.R. 611.2c). Thus, the only element in the set will gain double strike "until end of turn" (even if it's neither attacking nor enchanted nor equipped), and other creatures won't gain double strike this way (even if they're attacking, enchanted, or equipped).

Protection from creatures means, among other things, that the permanent or player with that ability can't be the target of abilities from creature sources wherever those sources are (C.R. 702.16b). This is an exception to the general rule that the term "creature" means a creature permanent on the battlefield (C.R. 702.16a; see C.R. 109.2 for the general rule).

EDIT (May 26): Add rule citation.
EDIT (Jun. 14, 2022): Edit second sentence.

Angel of Serenity
Razaketh, the Foulblooded

After the Angel's ETB trigger has been put on the stack and its targets have been selected, players can respond to that trigger. If you respond by removing the Angel somehow (for example with Razaketh's sac ability), its trigger is still on the stack waiting to do its thing. But its other trigger is now put on the stack, too, and as always, it is put on top of the stack, so somewhere on top of the other trigger. This means, that the return trigger resolves first, but there is nothing to return, because nothing has been exiled yet. The exile trigger will still resolve (because abilities on the stack exist independent of their sources) and do its thing, exiling all its targets that are still legal. In order to return those cards now, the Angel has to leave the battlefield, but it's not even there anymore, so the trigger can't go off. Even if the Angel returns and then leaves again, the game considers it a new object and thus its return abilty won't refer to the previously exiled cards since they were exiled by a different Angel as far as the game is concerned.