[Shangrila00]

If you have sages, every year a bunch of them will get diseased at which point they might as well be used to possess something. Occasionally, you'll get a diseased astral Black Priest. And sure, recruiting an Illuminated One solely for possession costs a fort turn, but so does recruiting a thug for nations that have them. It'll be worth it if the result is good enough. Not sure a Demon Knight would be worth it, but there should be something useful.