Several centuries prior to the birth of humankind, the Eternal known as Mentor created ISAAC - Integral Synaptic Anti-Anionic Computer to aid him in rebuilding and running Titan.
ISAAC initially was a gargantuan computer, both teacher and servant, which lays below Titan.
ISAAC was created by Jim Starlin and first appeared in Iron Man #55 (the same issue which features the first appearances of Thanos and Drax the Destroyer).
Major Story Arcs
When Vision was badly injured during an adventure in the Negative Zone, Starfox sought the aid of ISAAC in helping to repair and fix the Avenger. In the process, ISAAC and Vision's minds became linked and ISAAC effectively replaced Vision for a period of time. While influencing Vision, ISAAC took over all of Earth's computers and proposed the formation of a second group of Avengers in an attempt to make Earth a paradise similar to Titan.
Vision was eventually freed from ISAAC's influence and he and the Scarlet Witch quit the Avengers shortly afterwards.
For more information see: Maximum Security
During the Maximum Security crossover, ISAAC attempted to remove the moongem from the former supervillain Moonstone deeming her unworthy of it's power. The Thunderbolts and Captain Marvel fought ISAAC in defense of their team-mate. Eventually ISAAC rescinded and allowed Moonstone to continue to use the gem while she sought redemption with the Thunderbolts.