Auormo, Apex of Vitality

Mutate {w/b}{g} (If you cast this 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.)
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Whenever this creature mutates, you may
sacrifice a creature. If you do, you gain life
equal to its toughness, then put that many
cards from the top of your library into
your graveyard.