Mongoose Publishing's d20 RPG about Hell itself, consisting of 3 hardcovers.
"Play as Mortal, Demon or Fallen Angel. Equip yourself with the latest infernal weaponry such as the flayer or hellcannon and trap a demon within your chosen tool of warfare. Develop and nurture your mutations as they twist your flesh and increase your demonic power. Feed from damned souls and bind other demons in unbreakable Covenants. Do unto others before they do unto you and remember...
Conquer or suffer. These are the only choices in the Infernum."