In the Episode 8 “What If… Ultron Won?”, the nexus is that Ultron, after obtaining Vision's Mind Stone and vibranium body, defeats the Avengers and launchs a global nuclear holocaust, and goes further by gradually destroying other universes to put an end to all life.

@What If...? | The movie is shown on Disney+.

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Invincible Ultron with 6 Infinity Stones

Thanos appears on Earth to find the Mind Stone in order to complete the Infinity Gauntlet, but Ultron kills him instantly and takes away the 5 Infinity Stones, thus gaining control of all 6. Thanks to the 6 powerful stones inlaid on his forehead and chest, Ultron has unparalleled strength, using them to create a giant robot army and destroy all life across the universe.

Captain Marvel Carol Danvers, Clint Barton, and Black Widow Natasha Romanoff are the sole survivors on Earth. Danvers fights Ultron but eventually dies at his hands. Meanwhile, Barton and Romanoff find a copy of Arnim Zola's consciousness and attempt to bring it into Ultron's mind to destroy him. Barton sacrifices himself to let Romanoff and Zola, now in the body of a robot, escape from Ultron.

The upload failed because Ultron's main body was no longer in their universe.

The Watcher vs Ultron

Ultron discovers the existence of the multiverse and attacks The Watcher. They fight across the multiverse until Ultron has the upper hand and The Watcher is forced to flee. Ultron stands in The Watcher's place and now can observe the entire multiverse happenings.

The Watcher reluctantly asked the Ultimate Strange for help.

This special episode of the series shows the connection of the 3 Universes: in addition to the current universe is the universe of Strange the Supreme which was completely destroyed due to his undoing the Absolute Point in Episode 4, and the other universe where Thor meets Jane in Episode 7 and they confront Ultron in the final seconds of the episode.

And this is also the first time The Watcher has been forced to abandon its "non-interference" principle.