Crovax the Cursed enters the battlefield 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.
: Crovax gains flying until end of turn.
10/4/2004 Crovax can sacrifice itself to its upkeep triggered ability.
10/4/2004 You can only sacrifice one creature to it each turn.
10/4/2004 If you choose not to sacrifice a creature and he has no +1/+1 counters, then nothing bad happens. Note that some other effect must be keeping him alive if he has no counters.