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 (This creature can't be blocked except by creatures with flying or reach.) until end of turn.