This constellation actually happened when I (PLAYER A) drafted Ikoria on MTG:Arena yesterday, but I was unsure whether it would work in my favor. And therefore I didn't try.

PLAYER B has a Cavern Whisperer on the battlefield.

PLAYER A has a Lore Drakkis and Tentative Connection in hand.

Who will have control of the mutated creature at the end of a turn? I'm looking for the specific rulings regarding this topic. All I could find were the following rulings on the MTG Wiki.

The resulting merged creature has all of the characteristics of the topmost card or token, and it also has all abilities from its other cards and tokens.

The merged creature is the same creature it was before the merge, so any Auras or counters that were on it remain on it, it remains tapped if it was tapped, it's still attacking if it was attacking, and so on. If it's been under its controller's control since their most recent turn began, it can attack and {T}.


In this case, Cavern Whisperer won't be a valid target for mutation, because you are not the owner of Cavern Whisperer. This is per CR:

702.140a Mutate appears on some creature cards. It represents a static ability that functions while the spell with mutate is on the stack. “Mutate [cost]” means “You may pay [cost] rather than pay this spell’s mana cost. If you do, it becomes a mutating creature spell and targets a non-Human creature with the same owner as this spell.” (...)

108.3. The owner of a card in the game is the player who started the game with it in their deck. (...)

  • Nice. I never noticed that it said owner. So I guess that this means that you can mutate a creature that someone has taken from you too? – RBarryYoung May 26 at 13:59

While this specific scenario is invalid, as piotrek's answer explains, it is possible to alter it to make a valid scenario that preserves the main point of the question. Specifically:

PLAYER B has a Cavern Whisperer on the battlefield.

PLAYER A has a Lore Drakkis in exile stolen from PLAYER B by Thief of Sanity, and Tentative Connection in hand.

  • A casts the Tentative Connection targeting the Cavern Whisperer.
  • A gains control of the Cavern Whisperer until end of turn.
  • A mutates Lore Drakkis on top of the Cavern Whisperer.

There are plenty of other cards that can allow you to cast a card owned by the opponent, Thief of Sanity is just the first example that came to mind.

With this change, the owner of the Lore Drakkis is also the owner of the Cavern Whisperer, so mutating them together is valid. The question, then, is whether the combined mutated creature will return to PLAYER B at end of turn.

The relevant rules for this are:

702.140c As a mutating creature spell resolves, if its target is legal, it doesn’t enter the battlefield. Rather, it merges with the target creature and becomes one object represented by more than one card or token (see rule 721, “Merging with Permanents”). The spell’s controller chooses whether the spell is put on top of the creature or on the bottom. The resulting permanent is a mutated permanent.

721.2c Because a merged permanent is the same object that it was before, it hasn’t just come under a player’s control, any continuous effects that affected it continue to do so, and so on.

Rule 702.140c says it becomes a merged permanent and refers to rule 721 for how merged permanents work. Rule 721.2c then says it's considered still the same object, and that therefore any continuous effects still apply.

The Cavern Whisperer was a creature originally controlled by PLAYER B with a temporary control-changing effect. After the mutation, it is still "the same" creature, originally controlled by PLAYER B with a temporary control-changing effect. It will return to PLAYER B's control when Tentative Connection's effect expires at end of turn.

  • I think that if you steal a mutate creature card from your opponent and cast it for its mutate cost, it'd still only be able to target creatures you own, because you're the one casting it, and not your opponent. – nick012000 May 26 at 1:58
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    The mutate rules just specify that the owner of the mutate spell needs to be the same as the owner of the target creature. They don't say anything about the spell's controller. – murgatroid99 May 26 at 3:03
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    @nick012000 The mutate rules specifically state that the owner of the mutate spell and the owner of the target creature must be the same. Unless you're playing for ante, which is practically unheard of, there is literally nothing in the game that can change the owner of a card. The reminder text says "you" in place of "the owner of the mutate spell", but this is one case where reminder text is factually incorrect about a corner case for the sake of brevity. – Douglas May 26 at 3:42
  • Another similar example: Sally steals a creature from Juan using a non-expiring spell like Lullmage's Domination, Juan takes it back using an "until end of turn" spell like Claim the Firstborn, then mutates on it. The mutate is legal, but Sally still get the (now mutated) creature back at the end of Juan's turn. – BradC May 28 at 14:58

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