Ultron was a sentient robot programmed based on the brain patterns of Hank Pym. Ultron was an enemy of S.H.I.E.L.D. who often battled their agents such as Wolverine and Iron Man. When Dr. Doom created the Masters of Evil, Ultron became a founding member. Discovering that there were blue prints on the S.H.I.E.L.D. Omega Base that would greatly strengthen his weapon upgrades, Ultron sent Mysterio to steal the plans but he was stopped by the Ultimate Alliance.
Ultron was abandoned by the other Masters of Evil, and left to be destroyed with Baron Mordo by the Ultimate Alliance on their way to confront Loki and stop him from stealing the Destroyer Armor. He was presumably destroyed there.