Voidheart is a being of corruption. He looks like a stone elf with a crooked black 'X' across his face and black lips (occasionally oozing some sort of ichor).
As far as we can tell, he exists to bring corruption into the world. His preferred method of doing so is to get people (usually unknowing) to ask him for help with a problem, and he helps them, but in a way that defeats the entire purpose of what they wanted to accomplish and which also corrupts either them or the world in general. Voidheart and his three main servants each seek to corrupt a particular trait; The Apprentice corrupts magic, The Beast corrupts the body, The Crystalsmith corrupts the mind, and Voidheart himself corrupts the spirit.
The actual 'being' that we would probably call Voidheart is something from 'outside' that is possessing the body of a stone elf. He's been 'invited' into this world several times in the past, the stone elf is his most recent incarnation. We think destroying the body won't actually result in his/its death, just in it being sent back to where it came from to wait until it finds a way back again. In addition, destroying the body will probably result in a huge explosion of corruption that will taint wherever he is. (That has been the result of trying to destroy things that were corrupted by him/corrupted by his minions.)