So in Kilo and Sean's world, Voids are nasty little creatures; these goopy things can take control of monsters via liquifying themselves and entering in an unfortunate monster's mouth (not a pleasant experience). This will cause them to mutate and ooze some kind of green liquid from their mouths, causing some people to think that monsters have become infected with some wacky virus. There's no known cure for void-infected monsters, so doing a mercy kill seems to be the only route to '"free" them from Voids.

Voids are immune to physical attacks, but are weak to magic. They gain a weakness to physical attacks when they take control of a monster. No one currently knows where these things came from.