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 and realizing that the whole Star Child thing made about as much sense as Space Marines being all beakie space cops, 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. They have since been eradicated entirely.
In short, they were long since retconned and are unlikely to be making any further appearances.