In Demon: the Fallen, players assume the role of the rebel Elohim, angels who defied their Creator for love of humanity, and for their rebellion were cast into the Abyss, a prison hidden deep in the lands of the dead. Over the millennia, the Elohim have been twisted by their pain, anguish and suffering, manifest in the form of their Torment, becoming little better than demons in truth.
Now, as a great storm rages across the kingdoms of the dead, cracks have appeared in the walls of the Abyss, cracks small enough to allow the smallest and weakest of the Fallen to escape. Fighting both the force of the storm and the pull of the Abyss, they have made their way back to the material realm.
The only way they can remain there, however, is by taking a host - preferably a mortal whose soul was weakened enough to allow the Fallen to gain a foothold. The mortal soul is either thrown out into the storm, or is buried deep inside their own body, leaving the Fallen dominant.
The process of possession exposes the Elohim to the mortal's memories. In the majority of cases, this has no effect. A minority of Elohim, however, discover a deep-seated streak of humanity in their hosts, be it love, courage, ambition, curiosity, or even rage - something strong enough to shock them out of their Torment, to remind them of the angel they once were, even if only for a moment.
These human memories and emotions act as a bulwark against Torment, shutting out the memories of the Abyss, restoring the Elohim to some semblance of sanity.
PCs start out as one of this minority, given a second chance by the memories of their mortal host. What they will make of it, however, is another story... Source