DEMON:THE FALLEN is a role-playing game by White Wolf, set in the original WORLD OF DARKNESS setting. This game is about demons (obviously), but portrayed differently than in Judeo-Christian theology. Lucifer and the other fallen rebelled against God in order to love and serve humankind. There was war on Earth between the heavenly and fallen hosts resulting in great destruction and cruelty to humans, as the fallen used the humans they claimed to love as tools in their rebellion. Finally, the heavenly hosts were victorious and cast the fallen into Hell, a place of eternal isolation and nothingness. There, the fallen remained for all the aeons of earth's history.
And now all hell is breaking loose.
In DEMON, many fallen angels have managed to escape Hell, but it continually pulls at their essence. The only way for them to remain out of hell is to merge with a mortal body. The book implies that it is only possible to take the bodies of people with weak or broken souls; the brain-dead, the incurably insane, the suicidal, and those whose spirits have been crushed by the weight of a world of darkness. In this body, the fallen are very limited in their abilities; their memories of infinity and beyond have to fit in a mortal brain, and much is lost. Their fleshly shells are fragile, and the demons must continually search for faith to sustain themselves. They do retain some of their lore, their original ability to shape the universe, and their angelic form can be called upon at times. Whether they appear angelic or demonic depends on the fallen's level of torment, or how much they have succumbed to the dark side.
The book is exceedingly well written, with the first third of the book exploring the setting and abilities in narrative form - it makes for very interesting reading. I also found it interesting that some of the fallen have realized their error in rebelling against God and are in search of redemption (a position that surprised me given White Wolf's propensity for sticking a thumb in the eye of Christian belief whenever possible). I'm also surprised that people think that DEMON borrows from Judeo-Christian tradition when it is clearly Gnostic. It has a very Gnostic view of God, creation, Jesus, and humanity.
Many people (including myself) have wanted White Wolf to redo DEMON for their new setting. I think that they have chosen to replace it with PROMETHEAN: THE CREATED. It is billed as a "game of stolen lives", where a dead body is resurrected as a different creature, one with a mind but no soul. This new being searches for redemption, trying to understand humanity and to eventually become human. Prometheans even have Torment as a state they fall into over their suffering and have an "apocalyptic form" where their true selves are revealed. Many of the themes of DEMON seem to have been taken over by PROMETHEAN. If that is the case, it is an interesting decision by White Wolf.