Abaddon

The Corrupter. The Lord of the Abyss. The Devil. The god of demons. Abaddon is a somewhat controversial figure, for it is doubtful that he exists in the normal sense. He was named by the sage Quellan of Stavalot in the 8th century, who listed all things and the gods whose domain they belonged to. No god, he argued, was responsible for devils and demons. As all things must have a god, there must be one of devils, only one so horrid so that none would speak of it. The name Abaddon itself dates back from before the Third Empire, where it appears in texts speaking of a powerful being ultimately responsible for the downfall of the Second Empire, also called the Corrupter. 
Though there are numerous cults practising devil or demon worship, none are known to worship Abaddon. In fiction, however, he occurs often, though here he almost always referred to as the Devil.