Illuminati (40k)
This article is not about the alleged cabal which allegedly influences the course of history. Not that such alleged cabals exist. fnord
The Illuminati (meaning "illuminated") are a group whose members "illuminate" themselves by voluntarily allowing daemonic possession of their bodies and then exorcising themselves by sheer personal willpower, much like the Exorcists do. This makes them invisible to daemons (which raises the question of why they call themselves "illuminated"; maybe it's a joke on their part). In the Realm of Chaos sourcebook (from Warhammer 40,000: Rogue Trader), they were said to be like the Harlequins of the Eldar, running around in the shadows to round up the Emperor's descendants, the Sensei, sacrifice them at the Golden Throne while killing the God-Emperor of Mankind, and causing him to reincarnate into the Star Child.
In the third edition, as part of general grimdarkification, Games Workshop said that the Inquisition had decided that they were actually a Tzeentchian cult that plotted to kill the Emperor and plunge the Imperium into Chaos. That being said, in the Horus Heresy books it turns out that Malcador the Sigillite might have been a member. There was also a campaign run in Italy which involved the Illuminati and the Sensei somehow. The manual, available on archive.org, is in Italian, so we will need to spend some time with machine translation or get an Italian-speaking user to have a go at it before we can talk about what the event actually was about(I'm italian, I'm going to read that pamphlet and give you a brief summary, after a first look it appears there are rules for sensei and ronin, with followers, but I don't know for wich edition they are, if I find time I'll translate them on this page).
In short, this subject is an incredible source of skub among anyone even remotely interested in advancing the story in W40k, and will most probably not be resolved until GW makes a move.