Xaos

Xaos is the embodiment of all evil. Xaos is not a living creature it's a force of nature created from the negative emotions of humans, angels. and demons. Xaos took hold of Lucifer when he was "betrayed" by God and spread to several other angels, turning them into demons. Lucifer and the newly created demons abandoned God and were cast out of the heavenly domain and banished to Earth where they wreaked havoc among the humans. Lucifer and his demons were purified of their negative energy and returned to their heavenly state by the Guardians, thus ending the battle with the devil and ridding the world of evil. Xaos, with nowhere to go, left earth and presumably went to another planet.

Appearance
Xaos, when not possessing someone, appears to be a mass of black liquid. When someone is under the direct control of Xaos their face is completely shaded out and an outline of a mouth and eyes are the only things visible.