Ultron possessed the body of the robot Arsenal after the Avengers defeated Thanos. He was able to possess it due to a backdoor left open in its programming. Using Arsenal's energy absorbing powers, he completely drained the five Infinity Gems, while quickly defeating the Avengers, and left.
Iron Man stated to Captain America that he didn't know that Arsenal would be vulnerable to Ultron as he uses his Ultron Tracker App to track Ultron. Ultron makes his move by controlling a robot that was made by Hammer Advanced Weapons Systems. The Avengers head to a Hammer Facility facility where Ultron has taken control of the Dreadnoughts and a Crimson Dynamo Armor with Justin Hammer trapped inside. After disabling the Dreadnoughts, the Avengers head to an A.I.M. Island base where Ultron has taken control of the base's defenses and outsmarting Ultron Tracker App. Upon entering the A.I.M. base, Tony confronted Ultron who stated that the technology of A.I.M. and Hammer are his. Upon getting close, Tony placed the Ultron Destroyer Malware into Ultron in order to save Arsenal. This doesn't go well as Ultron escapes by setting the A.I.M. island base to self-destruct. 
Ultron takes control of a rebuilt Super-Adaptoid and uses it to attack Roxxon. The Avengers and Spider-Man had to fight an Ultron-controlled Super-Adaptoid. When Cap arrived, Ultron took control of the communications to have the S.H.I.E.L.D. drop team of Life-Model Decoy'S to prematurely drop as Ultron reprograms them into serving him. Upon tricking Director Nick Fury into sending unmanned drones upon the Avengers, Ultron abandons the Super-Adaptoid body. Back in Arsenal's body, Ultron raids Stark Tower where he hacks into J.A.R.V.I.S. and takes control of the Iron Man Armor's. Before Ultron can cause the Stark satellites into crashing onto every major city in the world, Iron Man blows up the Arc Reactor which causes Stark Tower to be damaged. After the Avengers are divided, Ultron sees this from outer space as Phase One of his plan is complete.
During the second phase of his plan, Ultron attempted to replicate himself onto the bodies of every living being on Earth. However, the once-fractured Avengers team regrouped and thwarted Ultron's plan. Concurrently, Iron Man attempted to fly Ultron into the Sun until Arsenal's programming briefly regained control. Arsenal used what time he had left to say goodbye to his friend Tony and flew directly into the Sun, just as a horrified Ultron regained control but melted in the extreme heat of the star.
Although his prime body was destroyed, a remnant of his programming still existed inside one of his old drones. A.I.M. was able to discover this sole-surviving drone through unknown means and used its parts to upgrade their Super-Adaptoids. However, Ultron's programming in the drone reactivated and absorbed the space metal also used on the Scientist's Supreme's new Adaptoids. Using this newfound ability, Ultron kills the new Scientist Supreme by absorbing his mind and intelligence, then morphed his body into a more threatening appearance.
After absorbing the remnants of the space metal, Ultron returns and once again wanted to replace humanity with robots, and starting it with the robot duplicates of the Avengers.
After being defeated and sent to the bottom of the sea, Ultron escaped and disguised himself as a human, Truman Marsh.
With this new persona, he became the government liaison appointed to lead the Avengers. Truman Marsh came to Avengers Tower declaring himself the Avenger's new boss, and showed instant distaste for the Hulk. Because of the release of the Terrigen Cloud, the President of the United States of America signed "The New Powers Act", giving the government control over the Avengers. He ended up replacing Hulk with Red Hulk being he was more focused on the lost Hydra data over the lives the Avengers saved.
When Truman recommended that all Inhumans become registered, the Avengers was vehemently opposed to such a system and cut all ties with the government. So, Marsh created his own team of Avengers, the Mighty Avengers. The team consisted of Red Hulk, Captain Marvel, Black Panther, Ms. Marvel, Vision, Ant-Man, and Songbird.
After a fight between the two Avengers' teams, Captain America got seriously injured and the main Avengers were arrested. The only member who avoided arrest was Black Widow, who combined forces with Medusa And Black Bolt to rescue the Avengers. After getting away, Inhumans all over the world started to loose control over their powers, because of the registration disks and it was at this moment that Ultron revealed himself as Truman Marsh.
Ultron then attacked the city with a robot army, but was eventually defeated by both the Avengers' teams. Seeing he had no other choice, Ultron possessed Tony Stark's nervous system through his arc reactor, knowing the Avengers wouldn't harm him.
It's unclear what happened to Ultron after the Beyonder had taken a section of the dimension into creating Battleworld. From what it appears, it seems Tony must have discarded and left his previous arc reactor to remain on "No Tech-Land" with Ultron still inside it.
Powers and Abilities
Ultron Prime: After possessing the body of Arsenal, Ultron now begins its campaign to defeat the Avengers using the possessed body of Arsenal. Robotic Body
- Superhuman Strength: Ultron is incredibly strong and can lift at least 100 tons
- Superhuman Durability: Ultron's outer shell is usually composed of Adamantium, rendering it almost totally impervious to damage. He is able to withstand concussive blasts and punches from Hulk
- Flight: Ultron has rocket boosters to fly
- Concussion Blasters: Laser channeled plasma beam with high and low density plasma options.
- Technopathy: Ultron can control any forms of technology, such as machines or robots
Ultron Prime II: After absorbing the remaining space metals from the super adaptoids of the Scientist Supreme, Ultron became much stronger
- Superhuman Durability: Ultron's new body became increasing damage resistant, capable of withstanding the missiles from Iron Man, Lightning Strike from Thor and Punches from Hulk, without going down
- Superhuman Strength: Ultron is incredibly strong and can capable of bringing down the Hulk with one punch
- Technological Manipulation: Ultron can manipulate technology in what he wants or wishes. For example, he used this against the Avengers, using their own weapons or technology against themselves.
Ultron is one of the world's greatest robotics experts, allowing him to easily design and build new bodies and weapons with his vast artificial intelligence.
- Voiced by Jim Meskimen.
- Ultron's initial design in the second season of Marvel's Avengers Assemble is based on that of Ultron during the Annihilation: Conquest story line.
- Ultron's appearances throughout the third season is based on that of Ultron from Avengers: Age of Ultron film.
- 19 appearance(s) of Ultron (Earth-12041)
- 2 minor appearance(s) of Ultron (Earth-12041)
- 14 image(s) of Ultron (Earth-12041)
- 5 quotation(s) by or about Ultron (Earth-12041)