You can also slowly let him transform into a demon (only the form, not the abilities). With his vanity he will surely a pain in the *** when he notices small lumps on his head where horns are starting to grow, or a lump on his back where a tail is growing. If you want to have fun, give him a slowly growing second head as well.
Another option is to give him savage fever of course!
Are you sure the PCs will really rescue him? :-)