There are plenty of X-Men who have the power to forever alter the fate of the future, even on a potentially multiversal scale. However, one version of the X-Men's Magik, the hellish New Mutant who was once Colossus' little sister, was so powerful that she was almost the key to Mephisto wiping out every version fo the X-Men, ever, across the entire Marvel Multiverse.
Now, we're taking a closer look at how one universe's Magik almost killed every mutant in the multiverse to save her son in 2002's X-Men Unlimited #37 by Kaare Andrews, Dave McCaig, Mike Kunkel, Joshua Middleton, Troy Nixey, Dave Stewart, Skottie Young, Pat Duke, and Randy Gentile.
X-Men Unlimited #37 hinges on Hutch. As the son of an alternate universe Illyana Rasputin, he was born not fully in his reality, or really any reality. Instead, he was essentially spread out between the various worlds of the multiverse all at once. This kept the boy away from his mother, forever separated from his family. Desperate to find her son, Illyana and an old man named Thatchel agree to a demonic deal with Mephisto. Hutch is given a real body and embraced by Thatchel, who explains his loss and his sacrifice: Thatchel's wife and son died at a child's birthday party when another boy there -- a mutant with pyrokinetic powers -- had his X-Gene activate. Unable to control himself, the boy's powers killed himself and twelve others, including Thatchel's family.
But by gaining a "son" in Hutch and a chance at revenge against all mutants for the act of one scared child, Thatchel completes his deal with Mephisto by using Hutch's powers to Mephisto's gain. Thanks to his connection to the entire multiverse, Hutch can be used almost as a lure, drawing every mutant from across the multiverse to one reality -- Earth-616. Driven into a rage, the various versions of the X-Men go to war with one another in an absurd and violent version of Spider-Verse. Above the chaos, Illyana and Thatch try to explain to Hutch that this is the only way for them to all be together. But the assembled versions of Charles Xavier -- who rose above the conflict to work together -- go to confront them, alongside with Marvel's main version of Shadowcat.
The Xaviers reveal to Thatch that doing this will actually result in the destruction of every mutant in the multiverse, as well as potentially all of the Marvel Universe to empower Mephisto. Upon their demises, Mephisto will take these billions of souls into his hellish vaults with them. Mephisto unleashes an endless horde of demonic monsters, which prove too gruesome and brutal even for a multiverse's worth of X-Men. Soon, the souls of each of them are drained from their bodies, empowering the monstrous villain. Mephisto even casually reveals that Thatchel's son was actually the mutant who killed all those people by accident before burning Thatchel with hellfire.
The showdown's sole survivor, Shadowcat, escapes into Thatchel's reality, finding herself confronting the boy and his mother. Illyana strikes at Kitty, defending herself for opening the door so Mephisto could fully reach Hutch and enact his plan. But upon learning the truth, Hutch uses his powers to drag all the assembled mutants back to their home realities, preventing Mephisto from being able to actually claim the souls of anyone. His ultimate plan defeated by a boy's sacrifice, Mephisto retreats back into the depths of hell. It's a quiet and vital sacrifice from Hutch, and a reminder of just how powerful the devilish Mephisto really is. If it wasn't for Hutch and the good in him, every X-Man who ever existed could have just been fodder to help Mephisto extend his reign.