Just an add-on to my discussion of the Jaq Draco novels. These guys have, obviously, not been canon in a long time... but isn't that exactly the way they'd want it?
THE ILLUMINATI IN INQUISITOR
If you don't like the Illuminati, this topic is not for you. For those of us who do, here are some gaming rules that I have drawn directly from the original fluff (always a dangerous thing in 40K):
"The existence of the Black Library is known to only a few, and entry is allowed to even fewer individuals. The library's 'mind' defends itself from the weak who would misuse its knowledge by refusing entry to all except those who have acknowledged and tempered the Chaos within themselves. The immature, who are still vulnerable to the promises and seductions of Chaos, find that they are unable to pass through its gateway. As a result few have seen within the Library or read any of its books. Only two groups come and go at will: The Human Illuminati and the Solitaires of the Eldar Harlequins."
- Realms of Chaos: Slaves to Darkness page 215.
"They are close-mouthed to such degree that an Inquisitor-Illuminati would torture a brother Illuminati to death to preserve their shared secret - and that brother would go willing to his death in such circumstances. For the Illuminati, the ends - the preservation of Mankind - justify any means, up to and including the destruction of the Emperor and the Imperium."
-- Realms of Chaos: Slaves to Darkness page 216.
IMMUNITY TO MIND CONTROL OF ANY TYPE: These sections seem to indicate that the possession by daemons that the illuminati have undergone has so hardened them that they would be invulnerable to illusion, mind control or any other power that directly effects the mind. They have been suckered once by Chaos, and are no longer buying into it, so Chaos, and immature psykers, cannot effect their minds. Similarly, no more brutal form of mind control, like torture, can ever effect them.
For the same reason, the Illuminati can never be re-possessed by daemons or other warp entities.
"But the Sensei have been duped. There is to be no Armageddon, no Ragnarok, no last battle fought against the forces of Chaos. At the moment the Emperor's will breaks, the Illuminati will offer up the remaining Sensei in a sacrifice to the Golden Throne of Earth. The Emperor and the Sensei will be united, in just the same manner as the Emperor has absorbed countless psykers. The new, evolved Sensei-Emperor, father and sons, will arise to protect and lead Mankind. Such is the plan of the Illuminati."
- Realms of Chaos: Slaves to Darkness page 216.
INFLUENCE OVER THE SENSEI: This seems to indicate that the Illuminati have some control over the Sensei, simply because the Sensei are firmly ensnared within their plans. I would say that a Sensei would have to take a Nerve/Cool test at a -30/-3 modifier to resist the orders of an Illuminatus, not because this is any kind of strange power, but just because he has been so thoroughly duped.
KNOWLEDGE OF THE WEIRD AND ARCANE: Further, their access to the Black Library would mean that the Illuminati know a lot of stuff that other characters just would have no clue about. Occasionally, this might manifest itself in some useful way. Give each Illuminatus a 1d6 roll at the beginning of each scenario to see if anything useful to him is around:
1-3: Nothing special.
4: A bolt hole of some kind exists. Somewhere on the board (within 1d20 inches of the Illuminatus, random direction) is an ancient teleporter, warp gate, secreted spaceship, or something else he can use to escape if he reaches it. Only the Illuminatus will be able to find the bolt-hole, and, if he does, he automatically escapes. He would not use it, of course, unless his mission were complete.
5: Some strange artifact or psychic item exists on the field. Maybe a force sword, force rod, daemon weapon, Chaos artifact of some type, etc. Only the Illuminatus can find and wield it, if he would. It's location is determined randomly, as above, and revealed to the player by the game master.
6: One basic model or mook level character on the other side is actually an ally. He is either an Illuminatus, Sensei or Harlequin Solitaire (equal chance of each). The Illuminatus player can switch this guy out for his 'true' self at any point during the game.
This knowledge also gives him the ability to detect any other Illuminated being, Sensei, Chaos creature or artifact, or psychic creature or artifact on the table, and to know what its powers are.
Illuminati Statistics and Abilities
Most Illuminati will be Inquisitors, and will have profiles and equipment as such. This is simply because the Inquisitor is the character most likely to be possessed by Chaos and have the strength to throw it back out. Other possibilities would include Witch Hunters, Navigators, Space Marine Chapter Commanders, Commissars, and ordinary possessed human psykers (or even non-psykers) who found the wherewithal to cast out the possessing daemon.
Remember that whatever the Illuminatus was before, he is now a servant of the Brotherhood only, and would never betray another illuminated being (including a Solitaire or other illuminated alien). Torturing one another to death where needed, does not count as betrayal, of course.
If you want to work it into the game, any time a character becomes possessed for any reason, or otherwise falls to Chaos, there is a chance that he fights off the daemon and becomes illuminated. The battle is instantaneous, and is resolved off the table in the realms of Chaos.
The daemon and possessed being fight with any psychic or magical powers and any psychic or magical artifacts they have available. The possessed character can use daemon or chaos weapons if he has them, and if they are not of the same alignment as the possessing daemon: Chaos is, of course, chaotic enough to struggle against itself. Non-psychic weapons and other equipment cannot help. They do not extend into the Empyrean for the purposes of this type of battle.
If the character wins, he emerges, scarred but triumphant, forever an illuminatus. If he fails, he is forever lost to Chaos.
Note that it is completely up to the GM whether or not this happens with any particular character. Most possessed beings give in to Chaos stupidly and wholeheartedly. Only a few have sense enough to try to break free before they are utterly consumed.