The Juggernaut is, in theory, unstoppable. There's nothing on Earth supposedly strong enough to put him down for good, so long as he carries the blessings and gem of the violent god Cyttorak. Over the years, he's proven how strong he is by holding his own against Marvel's mightiest heroes and villains alike.
In most timelines, this allows the Juggernaut to bounce back from all sorts of conflicts -- and even get a chance to become a hero. But in one reality, the Juggernaut became the last thing alive in the Marvel Universe. One early battle taking a far more disastrous turn for the X-Men created a version of the Marvel Universe where the Juggernaut became the last person alive -- and still quietly is tied to the X-Men's most dangerous modern foes.
The X-Men Villain Juggernaut Doomed An Entire Marvel Universe
Earth-9796 was the setting for What If...? #94 (Jorge Gonzales, James Calafiore, Mark McKenna, Brad Vancata, and Chris Eliopoulos). In this timeline, the Juggernaut's first battle with the original X-Men took a far more vicious turn. Instead of the Fantastic Four's Human Torch arriving in time to help distract Juggernaut long enough to remove his helmet and leave him open to psychic attacks, Cain Marko successfully murdered his hated step-brother Charles Xavier and slaughtered most of his original X-Men. Without the X-Men to fight them, the Sentinels were able to continue on largely in secret. This meant by the time the world realized the full danger posed by the machine, the Sentinels were far more advanced and deadly than they would have been otherwise.
As in the Days of Future Past timeline, the Sentinels eventually turned against humanity -- killing everyone in their path, and eventually moving on from just mutants to all humans. The Avengers and Fantastic Four fell in battle, and eventually the Sentinels developed a lethal disease to wipe out any remaining pockets of survivors. Some of the few to escape this purge were Iceman, Magneto, and the Brotherhood of Evil Mutants. Allying with the last humans, they created a small hidden society -- ironically realizing Xavier's dream of humans and mutants coming to a lasting peace. The only other survivor was, tragically enough, Juggernaut. Protected by the Gem of Cyttorak, Juggernaut became the last living man on the surface, spending years hunting down the Sentinels and wiping most of them out with his bare hands more out of boredom than anything else.
Juggernaut Ensured the Victory of the Sentinels
But in the end, even the last survivors were doomed by the Juggernaut. After being driven nearly mad by isolation, an encounter with a dying Magneto led Juggernaut to discover the last city. Juggernaut's foolhardy attempt to reunite with humanity ended up exposing them to the same disease that had wiped out most of humanity -- effectively dooming everyone else left on the planet save for the Juggernaut. Ashamed, Juggernaut left them to their fates at their own commands, forced to deal with his guilt in the role he played towards dooming this version of the Marvel Universe. It's a brutal ending for one version of the Marvel Universe, one where all forms of life (save Cain Marko) end up wiped out and forgotten.
It's also a tragic reminder of the dangers that still very much exist in the core-Marvel Universe. Juggernaut is currently a hero again, working alongside his nominal nephew as a member of Legion of X. But the Sentinel threat is still very much alive. Orchis -- commanded by futuristic Sentinels like the Nimrod -- have been making moves against the mutant nation since it began, and the upcoming Fall of X will likely see them finally make a move against the X-Men. As glimpsed in the other lives of Moira MacTaggert, a Sentinel victory could quickly spread out and ensure almost all forms of organic life are effectively wiped out once and for all. Even just one unexpected victory could allow such futures to come to pass. The multivere's most tragic Juggernaut highlights how all it takes is one battle to change the fate of an entire universe -- and how easily the Sentinels could become strong enough to wipe out an entire Marvel Universe.