Ultron-Sigma

Ultron-Sigma is the main antagonist of the Capcom crossover fighting game Marvel vs. Capcom: Infinite. It is the result of a fusion between Ultron and Sigma from the Mega Man X series by using the Space and Reality Infinity Stones. Their goal was to merge the worlds of Marvel and Capcom and to destroy all organic life on the merged world by unleashing a cybernetic virus, and then rule the new world. Ultron-Sigma would eventually evolve into the ultimate, final form, named Ultron Omega, after using the Space and Reality Stones, along with the Soul Stone, since they has no soul, which drove they insane, and allowed the heroes to destroy them.

Role in Marvel vs. Capcom: Infinite
Death is visited by Jedah Dohma, who proposes an alliance to achieve equilibrium between life and death on both their worlds. Needing the six Infinity Stones to do so, Death deceives Thanos and Ultron into aiding her, granting Thanos the Space Stone and sending Ultron to retrieve the Reality Stone from Abel City. Sigma intercepts Ultron, and the two forge an alliance. They betray the others and use the Space and Reality Stones to merge the two dimensions and fuse themselves into a single being named "Ultron-Sigma". In order to wipe out biological life, they proceed to unleash an evolved form of the cybernetic Sigma virus that turns organic creatures to synthetic beings under their control. In an effort to stop, him, teams of heroes from both worlds are dispatched to find the other four Infinity Stones.

After gathering the remaining four Infinity Stones, Doctor Light, Iron Man, Spencer and Hawkeye build a weapon to harness the Infinity Stones, but during the process, Ultron-Sigma attacks Avengers Tower. In the carnage, Thanos is released and attacks Ultron-Sigma, cracking the Reality Stone and forcing him to retreat. As Chris Redfield’s team is pursued by Jedah Dohma's Symbiote, the heroes return to repel it. They use their Stones to destroy the Symbiote and the falling core, but are infected in the process. With hours left until they are turned, the heroes head for Xgard. While one group distracts Ultron-Sigma, the rest infiltrate Sigma’s laboratory to finish their weapon, the Infinity Buster. When Thanos learns of Death and Jedah’s partnership, he becomes enraged and betrays the heroes, creating a mechanized gauntlet that absorbs Ryu’s Satsui no Hado before departing. Dante returns to the Dark Kingdom and reclaims the Soul Stone from Jedah. He arrives in Xgard and pretends to surrender the Stone, but because they have no souls, Ultron Sigma is overwhelmed by the Stone and is transformed into Ultron Omega. The Soul Stone purges the Sigma Virus within Thor, returning him to normal. The others appear and install the Infinity Buster into X, who uses it to destroy Ultron Omega.

In the aftermath, the virus is neutralized, but because the Reality Stone was cracked, the universes cannot be separated again. The heroes agree to protect the new world and split the Infinity Stones between them to keep them safe.

Standard form: powers, abilities, and gameplay
As a fusion between Ultron and Sigma, and the Infinity Stones (the Reality and Space Stones) that they held, Ultron-Sigma obtained both of their powers and abilities. In regards to gameplay, Ultron-Sigma retains most of Ultron's original move set, with Sigma's teleportation ability and swordsmanship in the mix. Its Hyper Combos are the exact same as Ultron's, with the addition of an extra Level 3 Hyper Combo that allows Ultron-Sigma to summon a powerful meteor shower at will. With power over Space and Reality, Ultron Sigma is nigh omnipotent.

However, Ultron-Sigma's only weaknesses are:
 * Being electrified, just like all other machines.
 * The Reality Stone inside the fused machine being cracked, almost allowing Ultron-Sigma to be separated back to Ultron and Sigma. This is seen when Thanos cracks the Reality Stone on Ultron Sigma's head, allowing the fused robot to feel pain for the first time. To prevent the Reality Stone from cracking further, Ultron Sigma must recover the Stone's crack carefully and prevent it from being shattered.
 * Due to not having a noble soul and willpower (or any soul at all), Ultron Sigma becomes completely weak to the Soul Stone. Not only does the Soul Stone transform Ultron-Sigma into the ultimate abomination, Ultron Omega, it also nullified the fused robot's pain suppression and drove them to insanity. The Soul Stone also negated the Sigma virus, causing Sigma, who is immortal by that virus, to become mortal.

Final form: Ultron Omega
Ultron Omega is the final boss of Marvel vs. Capcom: Infinite. It is the second and final form of Ultron Sigma. This form was created when Dante acquired the Soul Stone, and it showed him that even with four Infinity Stones, he and the others still could not beat Ultron Sigma, so he opted to go rogue and do it his own way. Dante shows up for the final battle and actually outsmarts and tricks Ultron-Sigma into using the Soul Stone, which Dante handed over to him prior. When Ultron Sigma attempts to use it to enslave his enemies, his fusion became more unstable, as Dante informs him that only a noble soul can use the Soul Stone. Ultron Sigma then used the full power of the two stones in their possession and mutated into Ultron Omega, due to the Soul Stone destroying his original body and becoming insane. This ultimately cured the Sigma Virus and removed Ultron Sigma's immortality, which allowed the heroes to use the Infinity Buster to finally destroy the fused villains for good.

Appearance
Ultron Omega is the upper half of Ultron (in the form of Ultron Sigma) attached to the head of Sigma, akin to Mega Man X5, with four floating hands: two claws and two fists. Sigma's head, which is slightly damaged, holds the Space Stone on its forehead and a cannon inside its mouth.

Gameplay
Ultron Omega is an unplayable boss that is similar to how Galactus works in Marvel vs. Capcom 3 and Abyss works in Marvel vs. Capcom 2, in which it is a giant boss that has its own part of the screen to itself and has its own health bar as Galactus, and is permanently unplayable as Abyss.

Ultron Omega occasionally summons Ultron Drones, and will sometimes back out and summon more Drones, during which it will regenerate vitality. Ultron Omega retains possession of the Reality and Space Infinity Stones, which it can use alternatively. Once one of the Infinity Stone's Infinity Storm is used, it switches to the other Stone and back. Ultron Omega can fire three tracking projectiles when in possession of the Reality Stone instead of one. The Ultron half can also throw its claws at the player, as well as attack with a flurry of energy beams from its palms. When enough damage is dealt, Ultron Omega will throw his hands in front of his head to block damage. The player must attack his hands enough to force him into a stun state where he takes triple damage.

Ultron Omega's Hyper Combos include:
 * Firing Sigma's blaster out of its mouth (level 1 hyper combo).
 * Grabbing the player, and trapping them in a paralysis bubble, during which Ultron Omega's fists and claws attack them, slam the opponent into the ground, and then finish them with a downward attack by Ultron's claws, similar to Wolverine's Drill Claw special move (level 1 hyper combo).
 * An enhanced version of his Level 1 blast attack, in which both Infinity Stones are used to channel a powerful beam of energy that can deal major damage to the player health bar (level 3 hyper combo).

Trivia

 * 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.
 * This is not the first time that Ultron has fused with another character; for example, recently, in the comics, Ultron has fused with his creator, Hank Pym.
 * Ultron Sigma can be considered to be a counterpart to Dark Kahn from Mortal Kombat vs. DC Universe, a fusion of two supervillains from two alternate universes and two different companies, and who spreads a negative influence, the Sigma virus, similar to how Dark Kahn spread the Kombat Rage in Mortal Kombat vs. DC Universe. (Note: Midway Games, the original developer for the Mortal Kombat series of video games, was bought out by Warner Bros., which also owns DC Comics, and was rebranded as NetherRealm Studios.)
 * Its leitmotif incorporates elements from Sigma's battle theme from Mega Man X2/X6 as well as "Heroes/Avengers Unite" from Avengers: Age of Ultron's original soundtrack.
 * In the second-to-last battle of arcade mode, the standard form of Ultron-Sigma is fairly easy to beat. However, in the first battle of story mode, Ultron-Sigma has a few "cheats," which make him very hard to play against, similar to the final bosses from the 2D sprite era of the Street Fighter series, and also 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. The cheats he possesses include:
 * 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
 * Despite Ultron Sigma not having the capability to wield the Soul Stone, the defused robots can use the Soul Stone at ease outside of Story Mode. Additionally, a mod hack revealed that Ultron-Sigma can still use the Soul Stone with no repercussion in gameplay.