Kaervek, the Spiteful

If you control more than one Kaervek, each will give each other creature -1/-1. If these effects cause a creature's toughness to become less than or equal to 0 or the amount of damage marked on it, those creatures die at the same time that the “legend rule” remove your excess Kaerveks.

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