Whenever a creature an opponent controls dies, you may pay {1}{B}. If you do, return that card to the battlefield under your control. If it's a creature, it's a Zombie in addition to its other creature types. {1}{B}: Regenerate target Zombie.

Crovax enters with four +1/+1 counters on it. At the beginning of your upkeep, you may sacrifice a creature. If you do, put a +1/+1 counter on Crovax. If you don't, remove a +1/+1 counter from Crovax. {B}: Crovax gains flying until end of turn.