Seljarm, Apex of Horror

Mutate {b/g}{u} (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, target
non-Human creature gets -3/-3 until end
of turn. If that creature would die this turn,
instead exile it and put a +1/+1 counter
on this creature.