If Crovax, Ascendant Hero stops being white, it gives itself -1/-1.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to white creatures may become lethal if Crovax leaves the battlefield during that turn, and nonlethal damage dealt to nonwhite creatures may become lethal if Crovax enters the battlefield during that turn.

If a second Crovax, Ascendant Hero comes under your control, you'll put one into its owner's graveyard due to the "legend rule" at the same time that any creatures getting -2/-2 are put into their owner's graveyard for having 0 or less toughness.

Crovax's last ability can be activated only while it's on the battlefield.