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.

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.