WARNING: The following contains spoilers for What If...? Season 1, Episode 8, "What If... Ultron Won?," streaming now on Disney+.
In the world of "What If... Ultron Won?," the Marvel Cinematic Universe takes a hard turn toward death and destruction when Ultron achieves his perfect form. This results in him becoming a genuine multiversal threat, racing across realities with the intent to end life across all of them. The is a big problem here, though. Technically, given the established rules of the universe, he shouldn't be able to.
What If...? may have just made a huge change to one of Marvel's most powerful items -- and it could have huge ramifications for the future for the entire multiverse going forward.
In the original Marvel Comics, the Infinity Stones -- originally known as the Infinity Gems -- were indeed powerful enough to shape and change the reality around them. But there were consistent aspects that kept them from being too powerful. Notably, Infinity Stones lack any power outside their home reality. Due to their fundamental connection to the reality they hail from, they could only be operated within their own reality. DC's Darkseid discovered as much during JLA/Avengers, and this became a major plot point during the Incrussions storyline that led into Secret Wars. The MCU seemed to be working on a similar rule, with the Infinity Stones proving capable of being moved across their own timeline but still operational, as opposed to how they became inert in the other-dimensional TVA base, as seen in Loki.
But What If...? just threw that rule to the side in the eighth episode of the first season, "What If... Ultron Won?." In the variant reality featured in the episode, Ultron claimed the body that would have otherwise become Vision. Uploading himself to the new Mind Stone-empowered body, Ultron quickly unleashed a war that killed most of the Avengers and subsequently wiped out most of humanity. In the silence that followed, Ultron was confronted by the Thanos of this reality -- who'd already gained the other five Infinity Stones. Ultron quickly killed Thanos and took the Stones for himself, making himself the most powerful force in the universe. He quickly traveled the cosmos and wiped out all life. With his expanded awareness, he witnessed Uatu the Watcher from outside reality.
Ultron then breached the walls that separate worlds and chased after Uatu, using his Infinity Stone-granted power to force the Watcher into an explosive duel. Primarily using the Power Stone to amplify his attacks, Ultron quickly proved himself capable of holding his own against the cosmic figure. But the problem is, according to the pre-established rules of the MCU and the Marvel Universe, he shouldn't be able to. Once he left his own reality, this variant of Ultron shouldn't have been able to use the Stones or any of their abilities. Moreover, because his body is actually powered by the Mind Stone, and removing it would be tantamount to killing him, going into another reality should have simply resulted in Ultron collapsing, powerless.
Even Uatu seemed confused and shocked by Ultron's arrival, suggesting this sort of breach really shouldn't be possible. There's a potential that Ultron constructed a way to maintain power generated from the Stones, explaining why he only uses them for basic attacks instead of utilizing the more dangerous abilities of the Time Stone and the Reality Stone. Otherwise, it's an odd break of what has always seemed like a cardinal rule of the Infinity Stones. If this means the Stones work across realities, then an interested party with multiversal capabilities -- someone like the recently introduced Immortus -- could theoretically claim all the Stones in the multiverse and effectively become a god. Breaking the Infinity Stone rule could either radically alter the Marvel multiverse or is a mistake on the creators' part.
What If...? Season 1 premieres new episodes on Wednesdays on Disney+.