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. Basically the campaign is about a fight beetween the forces of the illuminati, those of the inquisition and those of Chaos. The purpose of the illuminati is to gather as many sensei as possible in order to sacrifice them (they are not very happy about that) to finally awake the Star Child. Chaos want to corrupt all of them (well not a great change). On the other hand the inquisition consider them all (illuminati and sensei) heretics worthy only to be purged (once again not a great change). Keep in mind that this campaign took place during the sixth edition so the sensei and the illuminati has already been retconed. The campaign can still be considered valid with the inquisition as the final winner....i guess. Nevertheless the campaign is still pretty awesome. The fluff is pretty good and well written and the rules for the sensei are rapetastic (they can be level 4 psykers with the stats of a chapter master with a lot of cool rules. And all of this for 200 point. 350 if you upgrade them at level 4). But prepare yourself for the most awesome thing. The sensei can take a retinue which can be compoused by a lot of alien species like eldar harlequin, orks, tau etc. Among these species there also the SQUATS (they have even Hatred against the tyranids, prove that the theory that they have been devoured by the space bugs may actually be true).
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.