In Black Panther: Wakanda Forever, Namor the Sub-Mariner makes his MCU debut as Atlantis goes to war with Wakanda, effectively laying the foundation for the formation of a new Illuminati in Earth-616.
Doctor Strange in the Multiverse of Madness' Illuminati appearance, though short-lived, was the first time for the MCU to tackle the concept of a superhuman/mutant council governing the world.
Earth-838's Illuminati is actually inspired by a much older and clandestine super team. Making their first Marvel Comics appearance in 2005's New Avengers #7, the original Illuminati was comprised of Iron Man, Namor, Doctor Strange, Reed Richards, Black Bolt, Professor Charles Xavier, and Black Panther.
Following the Kree-Skrull War, Tony Stark realizes that each of these heroes had information about the alien races prior to the war spilling onto Earth.Read more on screenrant.com