Ultron is one of the Avengers' greatest foes, and one of the biggest villains in the Marvel universe. With a history stretching back to 1968, Ultron is a well-established threat who's extended his reach deep into space and across time. In 2007, Ultron began what could be considered his "final" goal: integrating himself with organic life and converting all life in the universe to Ultron. He began by taking over Iron Man's armor, which itself was integrated with Tony Stark biologically.
After being defeated there, his consciousness was flung into the depths of space, where it came in contact with the Phalanx, a race of cyborgs created by a techno-organic virus. Ultron takes control of the Phalanx, and begins infecting organic life across the galaxy. After being defeated by the budding Guardians of the Galaxy and Nova Corps, he is once again left adrift in the universe. After a few more tangles with the Avengers, Ultron returns in Rage of Ultron, where the conflict with its creator Hank Pym comes to a head, and led to one of the most unlikely combinations in Marvel's history.
RAGE OF ULTRON
In the aftermath of Secret Wars, Hank Pym's life took a dark turn. He grew pessimistic and began to view robotic lifeforms as mere machines. In a battle with the Descendants, a group of androids bent on converting organic life to machines, Pym broadcasts a signal to shut down every Descendant. The Avengers call him out, with Vision in particular questioning what Pym thinks of him if he can be so callous with other machine life.
Around the same time, Ultron returns to the solar system and begins converting organic life to Ultron drones with a nanotech virus. Naturally, he makes his way to Earth, where once again the Avengers step forth to oppose him. Ultron infects many of Earth's heroes, and Hank debates using the device against him to defeat Ultron once and for all.
Hank eventually confronts Ultron face to face, where Ultron claims Hank subconsciously created him to destroy the world. He plays upon Hank's fatherly instincts, luring him close enough to finally merge with his creator.
Now more powerful than ever, the new Ultron/Pym hybrid, or "Pymtron" handily defeats the Avengers. He is ultimately defeated by the Eternals' Starfox, who uses his love powers to force Ultron, now with a human heart, to finally love himself. The weight of his newfound emotions overcomes him, and he flees into space to grapple with them.
PYMTRON'S SECRET EMPIRE
After Rage of Ultron, Pym and Ultron appear to form a kind of peace and begin acting as a hero in space. They eventually return to Earth, and are confronted by the Avengers once again. While he eventually presents as Hank working together with Ultron, Wasp determines that Ultron has actually taken complete control and is using Hank's face to fool his former friends. He is defeated and flung into the sun, although he survives and Hank and Ultron continue their internal battle.
By the events of Secret Empire, Ultron has apparently set up a base in an unidentified forest on Earth. He also appears to have finally made peace with Hank, although in the process Hank has turned to full villainy. They somehow got their hands on a fragment of the Cosmic Cube, which attracted the attention of Steve Rogers' Hydra and the Avengers rebels led by a Tony Stark AI.
Upon their arrival, he forced them to sit down to dinner, saying that the Avengers has become less of a family. He eventually gives the fragment to the Avengers, although he agrees to not interfere with Hydra's plans.
Infinity Countdown saw a mad scramble by forces across the Marvel Universe to claim the reconstituted Infinity Stones. Ultron becomes one of the competitors, seeking the Soul Stone in space while sending his converted drones to claim the Space Stone on Earth. While he fails to gain the Space Stone, he manages to claim the Soul Stone by killing its first wielder, Magus. As he claims it, however, the fragment of his soul belonging to Hank Pym is ripped away and sent to Soul World, where it is devoured by the monster Devondra.
Ultron then plans to use the Soul Stone to spread his virus across the universe. He is ultimately stopped by Galactus, who retakes his World-Eater powers to destroy the planet where Ultron has made his base. Although Ultron escapes, he loses the Soul Stone and is wounded in the process. His current whereabouts are unknown, as is the status of his connection to Hank Pym.