Rule 730.2 · Merging with Permanents
To merge an object with a permanent, place that object on top of or under that permanent. That permanent becomes a merged permanent represented by the card or copy that represented that object in addition to any other components that were representing it.
Subrules
A merged permanent has only the characteristics of its topmost component, unless otherwise specified by the effect that caused them to merge. This is a copiable effect whose timestamp is the time the objects merged. (See rule 613.2.)
As an object merges with a permanent, that object leaves its previous zone and becomes part of an object on the battlefield, but the resulting permanent isn’t considered to have just entered the battlefield.
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.
If a merged permanent contains a token, the resulting permanent is a token only if the topmost component is a token.
If a merged permanent contains face-up and face-down components, the permanent’s status is determined by its topmost component. If a face-down permanent becomes a face-up permanent as a result of an object merging with it, other effects don’t count it as being turned face up.
If a merged permanent is turned face down, each face-up component that represents it is turned face down. If a face-down merged permanent is turned face up, each face-down component that represents it is turned face up.
A face-down merged permanent that contains an instant or sorcery card can’t be turned face up. If such a permanent would turn face up, its controller reveals it and leaves it face down. Abilities that trigger when a permanent is turned face up won’t trigger.
If a merged permanent contains a flip card (see rule 710), that component’s alternative characteristics are used instead of its normal characteristics if the merged permanent is flipped.
A merged permanent is not a double-faced permanent even if it contains one or more double-faced components. If a merged permanent contains one or more double-faced components that can transform (see rule 712), transforming or converting that permanent causes each of those double-faced components to turn so that its other face is up.
A face-up merged permanent that contains a double-faced component can’t be turned face down. If a spell or ability tries to turn a face-up merged permanent that contains a double-faced component face down, nothing happens and that effect doesn’t change any of its characteristics or their copiable values.