The Cosmic Phoenix Force, Jean has led the X-Men while alive and been their most pressing inspiration when dead. The source of some of their most hard-won victories and most bitter defeats, Jean is an essential aspect of the X-Men's legacy, which is what made it such a surprise when she traveled back through time and revealed herself to be one of Marvel's most despicable mutant villains.
Jean's path to villainy is actually a little more complication than it seems. Following some exceptionally dark days for Marvel's mutants that left them split into factions, Hank McCoy traveled back in time and invited the original team of teenage X-Men to visit the present day, hoping that witnessing their dark fates might shock them into taking a different path. The original five X-Men agreed, but they quickly discovered they were unable to return to their native time, reg themselves to growing up in the modern day.
The adult Jean Grey was dead at the time, leaving her teenage self to grow up in a future where she'd already become a martyr to the mutant race. Quietly, teenage Jean began to rebel, quickly taking over as team leader from a shaken Scott Summers - who couldn't believe his own destiny as a quasi-villain - and unlocking her telekinetic powers at a far earlier age than her original self. Under the tutelage of former villains like Emma Frost, Jean became a much more invasive telepath, even outing fellow teen X-Man Bobby Drake as gay - something Drake's adult self had never shared with his allies in the X-Men. Though it wasn't foregrounded in the pages of All-New X-Men (by Brian Michael Bendis, Stuart Immonen, and others) in which these events originally took place, things came to a head in X-Men: Battle of the Atom, in which readers saw who this teen Jean would become if she never returned to her native time period.
Jean Becomes Xorn
Battle of the Atom - a crossover event which ran over the X-Men books of the time - saw the X-Men of the future travel back to the present to insist the original five were sent back to the past. If that already sounds complicated, it quickly turned out the future X-Men weren't who they claimed - instead, they were the future Brotherhood of Mutants, a villainous sect who had given up on Xavier's dream when Alison Blaire - the first mutant president - was assassinated in the act of taking office.
The future Brotherhood were a relatively small cadre led by the villainous brothers Charles Xavier II (the telepathic son of Charles Xavier and Mystique) and Raze (the bitter son of Wolverine and Mystique.) Jean Grey was the only other true believer - originally going by "Xorn" and wearing a familiar skull-shaped helmet - with an older Molly Hayes, Hank McCoy, and a Hulk-like Iceman mind-controlled into filling out their numbers. Their aim was simple: to force the original five to return to the past and prevent their own future, despite the wishes of the much larger team of actual future X-Men who eventually came back to stop them.
This older Jean's identity was originally a mystery, owing to her adoption of only pretending to be Magneto. Jean's only real connection to Xorn was the use of his helmet - a device based on Magneto's helmet design intended to prevent psychic incursion. For Jean, it was used to limit her powers, and yet this version of Jean Grey was still one of the most powerful ever seen in comics.
Jean's Dark Future
While Jean's connection to the Phoenix Force has always made her powerful, it's not actually an intrinsic part of her mutant abilities, which include immense psychic power. In her depiction in Battle of the Atom, it seemed that Xorn/Jean was free of the Phoenix and running on her own power, but it was still enough to fight off Emma Frost and the Stepford Cuckoos in Wolverine and the X-Men #36, and to defeat the combined efforts of Wolverine, Cyclops, and Quentin Quire (another powerful psychic who commanded the Phoenix Force at the time.)
Other feats included activating every offensive capability of a SHIELD helicarrier, including launching Sentinel units, and Jean was only defeated in battle after being worn down by the current, future, and original X-Men teams in X-Men: Battle of the Atom #2. Even then, she wasn't overpowered - teenage Jean simply managed to crack her mask, which turned out to be the only thing keeping her jaw-dropping power in check. "Xorn" died in an explosion of psychic energy, though Charles Xavier II later faked her resurrection in future attacks on the original five.
Readers actually learned more about Jean's dark future in All-New X-Men #25, by Bendis and Bruce Timm, among others. Chiding Hank McCoy for meddling with the time stream, the Watcher Uatu shows him some of the outcomes of his actions - good futures which are now impossible, as well as dark futures which may never come to .
Distressingly, one of these dark futures includes the natural progression of the teen Jean Grey's life if she remains in the present. Disillusioned by knowing that Xavier's dream doesn't come to even in her extended lifetime, Jean eventually turns to Magneto as a mentor (something which did later come to in X-Men Blue) and, her powers fully awakened at a much earlier age, becomes a powerful despot who subjugates mankind to her will. Uatu explains:
In your heart you thought whatever happens to Jean Grey now must be better than her original, horrible fate. Because you couldn't image anything worse than everything she had been through. You couldn't image that without Charles Xavier to teach her and help her mold her persona, without that anchor to tether her to her humanity, that eventually it would be others who brought out her worst. It would be Jean who decided that her powers were given to her for a reason. That, like Magneto told her, it was mutants' birthright to conquer.
Jean as Magneto
This is a comparison that comes up again and again in Marvel's depiction of Jean's Xorn persona - in Battle of the Atom, her battle with the original five visually echoes the cover of X-Men #1, with the group charging into battle against Magneto. Along with the Xorn mask and Jean's apparent commitment to mutant supremacy, this older Jean also seems like the most ardent believer of the Future Brotherhood of Mutants, with Charles Xavier II and Raze more interested in fostering a future in which they're personally better off than subjugating mankind.
Like many tragic future versions of Marvel heroes, Jean's Xorn persona makes it clear that her modern-day heroism is a choice. The original five X-Men aren't heroes because they're inherently pure, but because they had good role models and made hard choices in their young lives. Hank brings the original X-Men into the future because he has a nostalgic perception of the team's youth, but it's ultimately revealed that even Jean - the best of the X-Men - could go another way if exposed to different experiences.
Ultimately, the original five X-Men did go home before this dystopian future could come to , but there's a dark caveat the comics have yet to explore - the older X-Men later unlock the memories of their time-traveling adventure as teens. For Iceman, it helps him come to with his sexuality, and for Cyclops it pulls him back from the brink of righteous extremism. The ways in which Jean is changed by these forgotten memories is less clear, but given that her teen self was on the way to being radicalized against the human race, it may be that the modern version of Jean Grey has more Xorn in her than anyone has yet realized.
