the illuminati (AKA; the sensai and...various other things
)
are well known, mentioned in everything from consipracy shows, to the call of cthulu and warhammer 40,000 (theyre the ones who're attempting to revive the emporer as the star child, and begin the last war)
as chris said, theyve been gutted a few times (if memory serves the inquisition got a few hundred suspected illuminati)
basicly they do an awful lot of nothing trying to scare people, (and attempting to revive the god-emporer, yes, but that one wont happen for a few thousend more years) basicly the modern illuminati consists of a bunch of nutters who read 1984 and liked what they saw (a government who maintains order by denying the existince of the enemy except in a militaristic manner, who alter the past so irrevicably that its forgotten, and break the spirir of those who oppose them, so they dont die as martyrs-basicly anyway)
and have all the worldly power of a stone patatoe.
(by the way, if im "dealt with" in the near future, be sure to spread the word that the illuminati/sensai/emporers got me to break/break/feed me to the emporer to maintain the astronomicon)