A few years ago, I wrote a little piece for a creative writing class about a guy being led astray by his best friend. He was, however, led towards the open arms of a succubus, and only saved when the 'weird guy' who dropped out of high school turned up and arrested the succubus and human collaborator in the name of the Inquisition and Rome.
So now, I've just remembered it, there's a lovely theme for a game brewing in my mind similar to many of the default settings in d20 Modern - Department 7 would be the part of the Inquisition that the players work for.
I've also got some ideas about a Pax Dei and Pax Roma, the two treaties that govern the movement and actions of demons (and angels) on Earth. I know that the Inquisition would report to the Pope, but would be governed by a Synod, and at some point I want the word Quorum to show up, since I love that word.
I can already see how re-skinning low level monsters to make them impish can work, and how the setting can brew over nicely. After all, there's no Pax Pandemonia yet.