Running a demon should also be a call-back to the talking animals article last week. A fallen angel or an Earl of Satan’s Court will have a very different mindset, values, and goals that the average NPC.
I had a Corruption Demon show up in a long-running game of C.J. Carella’s Witchcraft. I’m fuzzy on the details now, but I think the PCs fought something that was possessed by it and it ended up bound to the office that one of the PCs had for his PI business. He toyed with the idea of using it as a receptionist, but its physical manifestations always quickly rotted before the eyes and was too off-putting to the clientele so it was banished. Oh, but how I hoped they were going to keep it around! The possibilities for betrayal could have been endless.