The Illuminati assembled after Black Panther discovered that another Earth was colliding with Earth-616. Reluctantly he called the Illuminati together to stop the collision. After long debate they decided that the only way to do so was to destroy the other Earth with the Infinity Gems, but had to find the Mind Gem that was in the possession of Charles Xavier before he died. Beast eventually recovered a memory implant put in his mind by Xavier, which asked him to take his place in the Illuminati, and where the gem was. As soon as Hank retrieved it, Namor, Black Bolt, and Captain America showed up and recruited him into their team.
How do the Infinity Gems Work?
The Infinity Gems, originally referred to as Soul Gems, are six immensely powerful fictional gems appearing in American comic books published by Marvel Comics. Whoever holds all six gems and uses them in concert, such as in the Infinity Gauntlet, gains virtual omnipotence and omniscience. In later story lines and crossovers, a seventh Infinity Gem is sometimes added to the primary six.
The Soul Gem first appears in The Power of Warlock, with all six appearing during the second Thanos War story line in an Avengers and Marvel Two-In-One annual, with Thanos harnessing their latent energies to power a giant gem to extinguish every star in the universe.
Death reveals herself to Thanos of Titan.