Sliver Splicemaster

Mutate {w}{u}{b}{r}{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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Each Sliver card in your hand has mutate. The
mutate cost is equal to its mana cost.
Sliver creatures you control have "Sliver creatures
you control have +X/+X, where X is the number
of times this creature has mutated."