What If...? saw an Ultron variant become the greatest threat the Marvel Cinematic Universe has faced yet. After merging with Vision's body and slaying the Avengers of his universe, Ultron took the Infinity Stones and spread his genocide across the galaxy.
Ascending to a new level of consciousness, "Infinite Ultron" became aware of the multiverse and nearly brought his reign of terror to other universes. The inspiration for this story can be found in the comics - crossover event Annihilation Conquest. Overseen by Dan Abnett & Andy Lanning, the series was the launching point for the modern Guardians Of The Galaxy and one of comic Ultron's deadliest plans.
10 The Multiple Eyes Felt Insect-Like
For starters, Infinite Ultron's design clearly took some inspiration for the Ultron of Annihilation: Conquest. One of the most obvious ways is in the head - most Ultron designs stick for a semi-humanoid look, but both Annihilation and What If...? saw a shift towards the insectoid.
They gave Ultron multiple sets of eyes. This is fitting, given both versions are essentially the "hive kings" of drone armies.
9 The Red Cape Is Straight From The Comics
Another distinctive design choice gives away the influence of Annihilation: Conquest on What If... Ultron Won? - specifically, the red cape that both versions of the android wear. Capes are usually more Vision's style, but as a testament to his new domain over the galaxy "The Great Ultron" took to wearing one throughout Annihilation.
Meanwhile, What If...? Ultron adopts a cape as soon as he uploads to his organic body - the body which belonged to Vision in the Sacred Timeline. Even after he crafts armor for himself based on his original body, he maintains a flowing red cape.
8 He Merges With A Cosmic Force
Ultron, adrift in space after the events of The Ultron Initiative (by Brian Michael Bendis & Frank Cho), comes in contact with cybernetic race the Phalanx. They attempt to absorb him into their hive mind, but by sheer force of will, Ultron takes control of them instead - from there, he uses his new army to ravage the galaxy.
In What If...?, Ultron likewise gains a new cosmic advantage, but a different one. Having already merged with the Mind Stone, Ultron quickly slays Thanos and takes the remaining five Infinity Stones for himself. This Ultron is thus an even greater threat than the one from Annihilation: Conquest, for he ascends to omnipotence.
7 He Becomes A Threat Beyond Earth
Ultron is the greatest enemy of the Avengers, a team with many cosmic members on its roster. Despite this, he tends to exclusively be an Earthbound menace. Annihilation: Conquest changed this - Ultron's galactic rampage sees him subjugate the Kree Empire and the heroes who foil him include none of his usual adversaries.
What If...? takes this a step further. Once the only thing left living in Ultron's native universe is metal, he invades the Watcher's domain and begins using this to spearhead a campaign of genocide across the Multiverse.
6 He Uploads Himself Into An Organic Body
While far away from Earth, Ultron's goal in Annihilation: Conquest is to prove himself superior to humanity and usurp his father, one Doctor Hank Pym. To do this, he requires an organic body - he selects Adam Warlock, who was after all genetically engineered as the perfect human specimen. Kidnapping the High Evolutionary, Ultron has the scientist transplant his mind into Warlock's body through the cocoon which originally gave birth to Warlock.
Age Of Ultron already took inspiration from Annihilation: Conquest, effectively combining Ultron's creation of Vision with his body-jacking of Warlock. The Cradle even resembles a purely mechanical version of Warlock's cocoon. In any case, it's no coincidence that the "one small change" that enabled Ultron's victory is that he succeeded in taking Vision's body for himself.
5 He Actually Kills Some Heroes
As dangerous as Ultron is, the revolving door of death in Marvel comics makes it hard for him to rack up a permanent body count. Annihilation: Conquest did remedy this a bit - he first slays Moondragon by plunging his arm through her chest, then kills the android Praxagora by hijacking her body then activating its self-destruct sequence.
This is nothing compared to the carnage in What If... Ultron Won? however. While in the Sacred Timeline, Ultron was only able to kill Quicksilver, this version kills every Avenger (Black Widow excepted), all of humanity, and ultimately his entire universe.
4 He's Felled By A Virus
Ultron had planned to clone Adam Warlock and inhabit an army worth of the hero. Alas, he barely gets any time in the original Warlock's body, for the Technarchy member Warlock (no relation to Adam) purges Ultron from Adam's body using a techno-organic virus - while Ultron survives, Adam is freed.
A similarly technological virus is used to defeat Infinite Ultron - the last remaining copy of Armin Zola's consciousness enters Ultron's body and tears apart his consciousness.
3 He Grows Immensely And Can Exist At A Giant-Size
Ultron may loathe his father, but his final form in Annihilation: Conquest takes inspiration from Hank Pym's days as Giant-Man. After being denied Warlock's body, Ultron hijacks a fleet of Kree Sentries and remolds them into a giant version of his normal body.
Concept art for Avengers: Age Of Ultron indicates that Giant Ultron was almost included in that film, with the form being the result of Ultron Sentries clinging together. While the idea was discarded, Infinite Ultron uses the cosmic power within him to grow giant-size during the What If...? finale.
2 He Requires An Alliance To Destroy Him
The Guardians Of The Galaxy have Ultron to thank for their existence. The now-famous team line-up of Star-Lord, Rocket Raccoon, Groot, etc. first came together to repel Ultron's invasion - the team's eponymous title was penned by Abnett & Lanning and explicitly spun out of Annihilation: Conquest.
Infinite Ultron likewise requires a team of Guardians to defeat him - the Guardians of the Multiverse, that is. Incensed by Ultron's threat to all iterations of reality, the Watcher recruits a team from across different timelines (who just so happened to be the stars of previous What If...? episodes) to destroy him.
1 He Fights A Captain Marvel
Phyla-Vell, daughter of Captain Marvel/Mar-Vell, is a cosmic hero who has used both her father's alias and the name "Quasar." She's also the lover of Moondragon - enraged by Ultron's murder of Heather, Phyla vows revenge against the machine. Sure enough, in Annihilation: Conquest #6, it's Phyla who deals the killing blow against Ultron with her Quantum Sword, saving Hala and the galaxy in the process.
When Infinite Ultron invades Xandar, he encounters the MCU's Captain Marvel - Carol Danvers. Unfortunately for Carol, Xandar, and the galaxy at large, Infinite Ultron was even more powerful than the one Phyla destroyed.