Background
Note: these are near as buggery my own notes that I have passed around the group concerning background to the game. I make no apologies for the fact that this all draws heavily on ideas gained from Mage: The Ascension. I originally got the germ of an idea from the Book of Mirrors, then expanded it. But that's all in the developer's notes, as they stand.
This reality diverges from the World of Darkness at one major point. The Nephandi incursion that would have basically allowed them to seize the reins of power worked. Their grotesque summoning that would have given them near total power over humanity assembled actually happened.
(Note: If you don't have any idea what I'm talking about, I recommend that you read the Book of Mirrors, among others. Basically, the Nephandi (Wyrm and Satan worshiping mages) attempted to summon a major league daemon a the end of WW II. In the 'normal' reality they were stopped by an alliance of the Traditions (read the 'good guys of mage') and the Technocracy (read Weaver mages). In Angel timeline, the summoning happened). [End of long, boring aside]
Because of this, take the Mage version of the world of today and replace the Technocracy with the Nephandi. That's it. Biotech is one of their favourite things. As a working imagery of the world, look at the relative tech level of Marvel Comics' Age of Apocalypse and replace metal with biotech. The Nephandi think that they have won. Despite some rebellious Sabbat Vampires, the last of the Garou and the isolated pockets of mages (techno and other), the Nephs have just about got this reality.
There's only one thing that they forgot: God. The Prescence, The Origin of All Thought, call him what you will. he looked on from Heaven, and in the mold of a Satan-style horror movie, just said (and I'm paraphrasing kinda heavily here) "OI! NEPHANDI! NOOOOOOOOOO!". Well, not him, but the four combined King Angels of the Pax Dei (The Pax Dei is the army of Heaven and the King Angels it's leaders) held a gathering of all the Angels in Heaven, and sent some Angels down to Earth, where they would bond with as-yet unborn children and manifest themselves on the child's eighteenth birthday. These Seraphim would be Holy messengers, out to rid the world of the Nephandic curse
That's all that I'm working with for the time being. We're looking at lots of biotechnology, technomages as aides, etc. E-mail me if you have any ideas for expanding this. The address is on the two main pages, but is reproduced below, for all of you who actually read this far (It's also in my sig on the main Angel page)
Inspiration
All of the titles below have been a great help to me. JLA: Paradise Lost helped a lot with the Hosts. the works of Terry Pratchett are often just good for a laugh.
Alighieri, Dante - The Inferno
Chaosium Games - Call of Cthulu, Mythos
DC Comics - various titles, esp. John Constantine: Hellblazer and JLA: Paradise Lost.
Lovecraft, Howard Phillips - Various anthologies, esp. The Haunter in the Dark and other tales
Milton, John - Paradise Lost, Paradise Regained
Pratchett, Terry - The Colour of Magic, and all other Discworld books