Emissary of Pain

Mutate {5}{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.)
When this creature mutates, put a -1/-1
counter of each creature, then target creature
gets -1/-1 until end of turn for each creature
on the battlefield with a -1/-1 counter on it.