Cassiopeia, Serpent's embrace

Lifelink, Wither (This creature deals damage to creatures in the form of -1/-1 counters)
Whenever you cast a spell from exile, Cassiopeia, Serpent's embrace deals X damage to another creature an opponent controls of mana cost X
where X is equal to the number of times this
ability has triggered this turn. If there are no
targets available, instead exile the top card of your
library. Until the end of your next turn, you may
play that card.