David Haller is the son of Professor Charles Xavier and an Omega-Level Mutant unlike any other. While in his "natural" state, Legion is an immensely powerful telepath on par with his father, David's mind contains countless split personalities possessing infinite power sets and abilities. David Haller truly is "Legion," as they contain many.
Legion has the unique ability to absorb minds and manifest new mutant powers to match the personalities. This means that, in theory, he has no upper limit to his powers. This puts him on par with the likes of Franklin Richards and the Scarlet Witch at the height of their power, able to bend reality, time, and space to his will--and all without a magic gauntlet.
10 Controlling The Age Of X-Man Prison
First on the list comes from the Age of X-Man event, which spawned several miniseries attached to the Uncanny X-Men title. This was an alternate reality controlled by X-Man, aka Nate Grey, and almost all X-Men, as well as their mutant enemies, were brought to the Age of X-Man. In a miniseries called Prisoner X, Bishop infiltrated the Danger Room Prison Complex inside the Age of X-Man and discovered its most dangerous and isolated prisoner: Legion. However, Bishop soon discovered that Legion was controlling the entire prison as well as altering the memories of the inmates. Even when at the mercy of X-Man and his fabricated world, David Haller maintained a modicum of control.
9 Merging With X-Man
Shortly before the creation of the Age of X-Man, almost every living member of the X-Men had gone to war with X-Man, and Legion was there to help the team. X-Man, another immensely powerful Omega Mutant, had laid out his plans to change the world for the better through his control and his Horseman of Salvation. Legion made an effort to stop this by merging with X-Man and locking away Nate in David's mind. This measure ultimately failed, but it once again showed the immense power at Legion's disposal.
8 Absorbing 100s Of Personalities Outside Of Time And Re-Entering Our World
After being dispersed by Bishop (for reasons we'll get into later), David was expelled from reality as we know it and into a realm called "No-Time." Despite this, he still came into contact with hundreds of minds and began absorbing them as well.
This increased his powers exponentially, and he was eventually able to return to Earth. No-Time was a death in all but name, but David Haller still showed his ability to conquer it and become stronger for it.
7 Consolidating Power
As previously explained, Legion's immense power comes in part from the countless personalities contained within his head. However, this partially locks away David Haller's potential in identities apart from his own and keeping them out of his complete control. That is--until he started merging those other personalities into his primary David Haller identity. This puts that power within his reach, not only showing how powerful he already at his disposal but how much more powerful he could become through focus and control.
6 Creating His Duplicates
Another shocking display of power from Legion came in the accidental creation of several physical forms for his personalities. Legion himself experiences physical changes whenever another personality takes over, but he had not yet created full new bodies for these personalities yet. David helped the X-Men retrieve these duplicates and absorbed them back into himself--and almost absorbed Rogue, too, by accident.
5 Removing Himself From Existence
Killing a god is almost as impressive as being one yourself, and Legion has essentially done both. David was involved with another powerful mutant telepath by the name of Blindfold. Together, they explored and hoped to treat the numerous mental maladies experienced by David, but they only discovered the depths of David's illnesses.
David foresaw that he was almost certainly going to kill Blindfold as well as many others. To stop this from coming to pass, David wiped himself and almost all memory of him from existence. However, Blindfold herself remembered David. Unsurprisingly, Legion has returned in the time since.
4 Wiping Out The Elder Gods Of Limbo
At one point, the Elder Gods of Limbo were unleashed upon Earth by General Charles J. Ulysses through the use of the Bloodstone Amulet, and the New Mutants worked to beat them back. However, the New Mutants alone were not enough to beat the Elder Gods, even with the help of Magik. As such, they brought Legion back to our reality to beat the Elder Gods, and he manifested his God-Mutant form to beat and destroy the Elder Gods of Limbo.
3 Creating The Age Of X-Man
Though it was not by his own design, Legion's powers were used by X-Man in the creation of his Age of X-Man reality as well as banishing the X-Men, David included, to that reality. Needless to say, creating an entire universe is an act of immense power, and it's not even the only time that David Haller has done such a thing.
2 Creating The Age Of Apocalypse
Indeed, Legion was responsible for creating the reality known as the "Age of Apocalypse." During a period where David was zealously committed to fulfilling the dream of Charles Xavier, Legion went back in time to kill Magneto. David believed that Magneto was the main thing keeping Professor X's dream from becoming a reality, but David instead accidentally killed Professor X in the past. This set off a chain reaction that led to the creation of the Age of Apocalypse, where the legendary En Sabah Nur took over Earth and the X-Men are left to be embattled resistance fighters seeking to bring Apocalypse down. This was only resolved by Bishop, who remembered reality as it was, forcing Legion to witness the consequences of his actions and creating a "psionic loop" that repaired the timeline and expelled Legion to "No-Time."
1 Creating--And Ending--The Age Of X
Another reality Legion has made--and this one was deliberate--was the Age of X. When Doctor Nemesis began removing personalities from David's mind, Legion manifested the Moira Kinross personality. This one crafted the new world dubbed Age of X. In this world, Legion is treated as a beloved hero and member of a superhero team called Force Warriors. Once this truth was exposed in the Age of X, David confronted Moira and repaired our world.