Midnight's Dreamthief

Mutate {2}{b}{b} (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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When this creature mutates, exile the top card of target opponents library. You may look at and
cast that card for as long as it remains exiled, and
you may spend mana as though it were mana of
any type to cast that spell.