|“||We are born of two universes, Each of them imperfect, each of them afflicted with the cancer of organic life. From their ruins, a new universe shall be forged, in the fire of the stones of reality, and all life remade in our image, in steel, free from the sins of flesh. We are Ultron Sigma. We are your god.||„|
|~ Ultron Sigma at the very beginning of Marvel vs. Capcom: Infinite's story mode.|
|“||You shall feel our wrath... KNEEL BEFORE YOUR GOD!!!||„|
|~ Ultron Omega at the second final battle of the story mode of Marvel vs. Capcom: Infinite.|
Ultron Sigma is the main antagonist of Marvel vs. Capcom: Infinite. He is the combined hybrid of both Ultron and Sigma. Simultaneously voiced by Jim Meskimen and Chris Tergliafera, the respective voice actors for Ultron and Sigma. His "voice" is that of both Ultron and Sigma's voices overlapping each other, owing to his being fused from the two characters. They later transform into their final form, Ultron Omega, for obtaining the Soul Stone that was being tricked by Dante.
After Death is convinced by Jedah into merging both the Marvel and Capcom universes in order to bring equilibrium between life and death, she calls upon Thanos and gives him the Space Stone, ordering him to find the remaining five, while pointing out the Reality Stone was out of reach. Thanos calls Ultron and shows him Abel City, where the stone is located, which was protected by a force field that only Ultron could cross, offering Ultron the city and all of its wonders in exchange for his cooperation. Using the Space Stone's power, the two reach the field's border. As Ultron crosses it by enetering Cyberspace, he is immediately confronted by Sigma. The two admired one another's mechanical natures, though Ultron disliked Sigma's pseudo-organic form, just as Sigma admit and considered his own form is nothing more than a mock-up of an actual organic lifeforms. Resolving to work together to "cleanse" both of their universes of organic life (of which they were both contemptuous), they move into Sigma's base on Abel City, where Sigma kept the Reality Stone. At this point, Ultron and Sigma began to betray Thanos, with Sigma had Ultron to stab his body and upload his A.I. into the former, merging together into a single being called Ultron Sigma. After defeating Thanos and taking the Space Stone, Ultron Sigma began use it and Reality Stone to not only transform Ultron's old body, but also brings about "the Convergence", fusing the Marvel and Capcom universes into one. From there, he took over Asgard (now fused with Abel City to become "Xgard"), turned the Asgardian warriors into cybernetic slaves, and even managed to turn Thor into his puppet, before the other heroes from both universes freed Thanos from Ultron Sigma and escaped with him.
After this, the heroes of the Marvel and Capcom universes work together to find the other Infinity Stones and stop Ultron Sigma. In the meantime, Ultron Sigma forges an alliance with the Grandmaster Meio, who works on creating a weapon inside Luanehnum's core using the Sigma Virus and the Power Stone. As the heroes separate to look for the Stones, Ultron Sigma sends his drones to track the heroes, eventually attacking the unprotected Avengers base himself. Iron Man, Spencer and Dr. Light use their combined efforts to shortcircuit Ultron Sigma, causing an explosion that frees Thanos. Thanos attacks Ultron Sigma and overpowers him, cracking the Reality Stone. This causes their fusion to become unstable, so they retreat to repair themselves. After the heroes destroy Meio's weapon and a huge symbiote monster created by M.O.D.O.K. and Jedah, they take the three stones in their posession and attack Xgard, while Dante goes after Jedah to retrieve the Soul Stone in his possession.
When the heroes finally reach Ultron Sigma, he reveals that Dante betrayed them and had already given him the Soul Stone. However, Dante had been tricking him into using the Soul Stone's power, blasting him away, since he had no soul and thus couldn't wield it. With their fusion becoming more unstable as a result, they transform into Ultron Omega and trap Dante for his trickery until Thor (now cured by the Soul Stone) and X manage to save the Devil Hunter from the unstable fused robot's grasp. Dante and Captain Marvel then attack Ultron Sigma while X charges a weapon powered by the other four Infinity Stones designed by Iron Man. With Ultron Omega defeated, X fires the weapon which ends the corruption caused by the Sigma Virus and can ultimately destroy Ultron Omega permanently. Yet, the two worlds are still in the converged state, as the Reality Stone is cracked due to the result from the previous battle of Thanos and Ultron Sigma at the Avengers Tower, rendering the stone useless and might cause another convergence with other different realities when being used in this state, thus required to be fixed in the mean time. Afterwards, the heroes each agreed to safeguard the Infinity Stones to keep them out of the wrong hands, and to keep fighting for their fused universes as they were.
- Ultron Sigma is the second original antagonist for the Marvel vs. Capcom series, behind Abyss, but it is the first fusion of playable characters in the series.
- Ultron Sigma can be considered to be a counterpart to Dark Kahn from Mortal Kombat vs. DC Universe, a fusion of Darkseid and Shao Kahn, two supervillains from two alternate universes and two different companies. (Note: Midway Games, the original developer for the Mortal Kombat series of games, was bought out by Warner Bros., which also owns DC Comics, and was rebranded as NetherRealm Studios.)
- His battle theme incorporates elements from Sigma's theme from Mega Man X2/X6 as well as the following tracks, "Heroes/Avengers Unite" from the original soundtrack of Avengers: Age of Ultron.
- This is not the first time Ultron has fused with another character; In the comics, Ultron has fused with his creator Hank Pym.
- In the first battle of Story Mode, Ultron Sigma has a few "cheats" similar to the final bosses from 2D era Street Fighter games, and particularly from SNK fighting games. Even if the player manages to survive long enough in the first battle without dying, the next scene will play out the same regardless:
- Enhanced defenses and an increased health bar
- Similar to the Mind Stone's Infinity Storm, its Hyper Combo Gauge keeps on automatically filling up to maximum
- An exclusive powerful Story Mode Level 3 Hyper Combo that allows Ultron Sigma to summon a meteor storm at will.