The group was revealed to have been formed very shortly after the Kree–Skrull War. Their history was discussed in the special New Avengers: Illuminati (May 2006). The Illuminati was established to exist (via story retcon) in their first published appearance in New Avengers #7 (July 2005), written by Brian Michael Bendis. The characters joined forces and secretly work behind the scenes. The Illuminati are a fictional secret society group of superheroes appearing in American comic books published by Marvel Comics.
Prominent members of the Illuminati (clockwise from left to right): Iron Man, Black Bolt, Doctor Strange, Mister Fantastic, Namor the Sub-Mariner and Professor X in artwork for the cover of New Avengers: Illuminati Vol 2 #1 (Feb, 2007).