In the 1990s, Legion traveled back in time to kill a pre-supervillain Magneto in a misguided attempt to please his father, Professor X. When Legion accidentally killed Xavier instead, he inadvertently created a dark new timeline called the Age of Apocalypse, where the X-villain Apocalypse ruled a dystopian world.
The fan-favorite "Age of Apocalypse" storyline took over the X-Men's world for several months in 1995 and 1996, where it offered bold new takes on familiar heroes and villains. While Cable is a grizzled time-traveling soldier in the Marvel Universe, he was the young Nate Grey in the Age of Apocalypse timeline.
Where Cable is the son of Cyclops and Jean Grey's clone, X-Man was genetically-engineered by the mad scientist Mr. Sinister, who used the genes of his world's Cyclops and Jean Grey. The ultra-powerful Nate Grey was one of the Age of Apocalypse's breakout characters, and his adventures continued on in the Marvel Universe after his home timeline was seemingly erased.
Throughout his 75-issue solo series, X-Man struggled to adjust to the Marvel Universe and deal with immense psionic powers that were slowly killing him. After befriending Cable, Cyclops and Jean Grey, joining them on a short-lived X-Men team, he evolved into a kind of mutant shaman. To defeat an alien that was trying to harvest Earth's energy on a cellular level, Nate essentially turned into a pure energy being and put a little of his power into every living cell on Earth in 2001.
When he returned a decade later, Nate had an encounter with Norman Osborn's Dark X-Men that left him in a drastically weakened state. After joining the adult New Mutants for a few adventures, he faded into the background of the Marvel Universe.
While he's been gone for the past several years, Nate will be a big part of the X-Men's immediate future. While X-Man appears to be central to the next several issues of Uncanny X-Men, he'll also apparently take center stage in the upcoming "Age of X-Man" crossover, which is due out in 2019.
This issue certainly positions X-Man as Marvel's strongest mutant, but there's still a little bit of room for debate. The Phoenix Force doesn't factor into this issue, and that can boost Jean Grey or someone else's powers to universe-shaking levels. Another one of Legion's multiple personas might have a different assessment of his own abilities. And Franklin Richards, the ultra-powerful adolescent mutant son of the Fantastic Four's Reed Richards and Sue Storm, isn't as strong as he will be once he grows up.
While those technicalities will let the debates rage on, Nate Grey can still make a very strong claim to being Marvel's strongest mutant, for the moment at least. Before "Age of X-Man" kicks off next year, X-Man will continue to wield his powers in Uncanny X-Men, which is currently being released weekly.