Meznem, Liquifying Wyrm

Flying, indestructible
If Meznem would enter, if it wasn't cast or no mana
was spent to cast it, exile it instead.
Whenever a non-token creature an opponent
controls dies, exile it. It gains "Mutate {0}". You
may cast it for its mutate cost for as long as it
remains exiled.
Whenever this creature attacks, creatures your
opponents control get -X/-X until end of turn,
where X is the number of times this creature has
mutated.