Mutate

Mutate is a keyword ability that is featured in Ikoria: Lair of Behemoths. It allows two or more permanents to merge.

Description
If you cast a spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.

Mutate has roots in Bestow, Emerge and Augment, acting like an ability-granting Aura spell. Whereas Bestow is more expensive due to the creature that remains, Mutate has lower costs and tends to award compounding mutations. It shares Bestow's bonus of allowing a mutator to enter the battlefield in the case of invalid targets, but like Augmented creatures, the entire stack of mutates will change zones together. Once off the battlefield, the component cards will separate, and if directed to return after a period, will all return as individual creatures.

The distinctive aspect of Mutate is that the final creature can be the size of either the mutating creature or the creature it targets, which means the battlecruiser magic of multiple Auras is not present. The mutation trigger and static sizing is something Mutate shares with Emerge.

Mutate was first referred to on the silver-bordered card Surgeon General Commander in Unsanctioned.

Rules
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Casting and resolving creature spells with mutate

 * Mutate represents an alternative cost. To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as the alternative cost of mutate), add any cost increases, then apply any cost reductions. The converted mana cost of the spell remains unchanged, no matter what the total cost to cast it was and no matter whether an alternative cost was paid.
 * A spell cast with mutate becomes a mutating creature spell. It requires a target creature with the same owner as the mutating creature spell. In the rare case that the player casting the mutating creature spell is not its owner, that player must choose a target creature the spell's owner also owns.
 * A mutating creature spell is a creature spell, even if it won't enter the battlefield as a creature.
 * As a mutating creature spell begins to resolve, check whether its target is still legal. If it's not, the spell ceases to be a mutating creature spell. It continues to resolve as a normal creature spell and enters the battlefield.
 * A mutating creature spell that resolves doesn't enter the battlefield. It simply merges with the target creature. As this happens, the controller of the mutating creature spell chooses whether to put it on top of the target creature or on the bottom. Abilities that trigger whenever a creature enters the battlefield don't trigger.
 * If the target creature is already a merged creature, the mutating creature spell can be put only on top or bottom. It can't be put in the middle.

Merged permanents

 * 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 undefined.
 * The characteristics of the combined merged creature are copiable.
 * The merged creature is a token if its topmost object is a token. If its topmost object is a card, it's a non-token creature, even if it contains a token.
 * If the merged creature has any abilities referring to it by name, such as that of Phase Dolphin, it means "this creature," even if mutating causes it to have a new name.
 * If a creature that's a copy of another creature mutates, the characteristics it gains from mutating apply after applying the copy effect.

"Whenever the creature mutates" triggered abilities

 * An ability that triggers "whenever this creature mutates" triggers when a mutating creature spell becomes part of the merged permanent. It doesn't trigger if a mutating creature spell becomes a normal creature spell and enters the battlefield.
 * An ability that triggers "whenever this creature mutates" triggers whether the ability came from the mutating creature spell or from the creature that the mutating creature spell merged with.
 * If a creature has multiple "whenever this creature mutates" abilities, its controller chooses the order for those abilities to resolve.
 * If an ability triggers when a creature mutates and counts how many times that creature has mutated, it includes the time that caused the ability to trigger.

Leaving the battlefield

 * A merged creature can't be split apart until it leaves the battlefield. For example, you can't sacrifice one creature card out of a merged creature if you're instructed to sacrifice a creature; you must sacrifice the entire creature.
 * If a merged creature leaves the battlefield, one object leaves the battlefield and each card is put into the appropriate zone. For example, if a merged creature containing three cards dies, one creature died and three cards were put into a player's graveyard.
 * If a merged creature is put into a library, its owner chooses the relative order of those cards. For example, if a merged creature containing three cards is put into its owner's library third from the top, those three cards are put third, fourth, and fifth from the top in the order of their owner's choice. This order isn't revealed to other players.
 * If an effect exiles a merged creature then returns it to the battlefield, the individual cards each return. They're no longer merged. If that effect has an additional effect on the returned object, it affects each of these permanents.
 * If multiple replacement effects could be applied to the merged creature leaving the battlefield or being put into its new zone, applying one of those effects affects all of the moving objects.
 * In the Commander variant, if a commander is part of a merged creature, the resulting creature is a commander. If it leaves the battlefield and the commander is moved to the command zone instead, that permanent and the other cards contained by that permanent go to the appropriate zone while the commander card goes to the command zone.

Unusual situations

 * If a merged permanent has characteristic-defining abilities, they overwrite the characteristics from the topmost object. For example, if a merged creature has an ability that defines a * in its power or toughness, such as that of Boneyard Mycodrax, that ability overwrites the power and toughness defined by the topmost object of the merged creature.
 * If a mutating creature spell merges with a creature that's only temporarily a creature, it will remain a merged permanent when the effect making it a creature expires. It still has all of the characteristics of the topmost object, which may cause it to be a creature or to not be a creature, and it has all of the abilities of its other cards.
 * The merged creature is face up if its topmost object is face up. If its topmost object is face down, it's a face-down creature, even if it contains face-up cards.