Belial is a character from the Painkiller series.
In Painkiller: Overdose players were able to take on the role of Belial - half angel, half demon - for the first time.
Daniel's actions free Belial from his prison, whereupon he begins to seek vengeance for his treatment.
In Painkiller: Hell & Damnation, Belial is introduced towards the end of the game where, thanks to the actions of Eve, he joins forces with the series protagonist Daniel Garner in order to battle the forces of Hell and, most notably, the horsemen of the apocalypse.
