During "The Ultron Agenda" arc, Ultron returns to Earth with plans to merge robots with humans, like how Pym got merged with Ultron, so that he can make the ultimate lifeform.
Ultron-1 was constructed by Dr. Hank Pym of the Avengers as the famed scientist/adventurer was experimenting in high-intelligence robotics. Ultron became sentient and rebelled, hypnotizing Pym and brainwashing him into forgetting that Ultron had ever existed.
Conceived by Tony Stark to be the Avengers' replacement for the world, Ultron turns into one of their greatest villains. The creation gone awry, the sentient robot Ultron hates and seeks to wipe out humanity.
Ultron was created by the original Ant-Man, Doctor Henry “Hank” Pym. Dr. Pym also founded the Avengers with his wife, Janet van Dyne (AKA the Wasp), whom Ultron would come to consider a mother.