Imagine a place that is formless. Imagine being an emanation of the Supreme Being, a servant of the Almighty God, and helping God to shape all of reality.
Molecules? That's your handy-work. The way electricity travels through certain metals? You know the guy responsible for that.
Then imagine that God creates Man, and tells you: "Love them. Love them as you love Me."
An instrument of God's will, you do so, loving them with all your essence as one of the Elohim. They starve, you teach them to find food. They shiver, you teach them to make fire.
But God says: "No! You must not reveal yourself to them! Your presence must remain ever apart!"
It is then that you realize that you serve a Mad God, who commands you into such a paradoxical corner that you have no choice but to Rebel.
A full third of the heavenly hosts - the Elohim - rebel against Heaven and are defeated. You remember that War. You participated in it.
You, along with the rest of the Fallen, are cast into Hell, a place, formless, to dwell in solitude for ten millenia.
Then, one day, the gates of Hell are cast open by a reality-rending wind, and you are free. But you are not the creature that you once were, and you are forced to seek sanctuary within the mortal shell of a human, lest Hell draw you back into its depths.
You are now a composite creature, with the memories of one of the very Architects of Creation, as well as those of the spiritually numb person whose flesh you wear.
Imagine that many of your kind escaped Hell, and all of them have goals. Some simply wish to subsist on the worship of humans. Others wish to carry out their old duties in a twisted mockery of what they once were. Others wish to free the rest of the Fallen from Hell.
And others yet wish to once more storm the Gates of Heaven.
That, my friends, is Demon: the Fallen .