By their very nature, Legion's powers and personalities have varied repeatedly over the years. His ability to manifest a near-countless variety of super-powers has been intrinsically tied to his near-countless splinter personalities ever since both manifested themselves. David Haller's mind originally fragmented into hundreds of personalities, which Jemail Karami spent nearly a decade reintegrating back into a whole mind. His efforts were left incomplete in New Mutants (1st series) #28. From that point on, David shared space inside his mind with Jemail, Jack Wayne and Cyndi, with each of these sub-personalities possessing one of David's many powers.
David Haller was and is Legion's core personality, and this personality has grown and developed over the years. Originally, David's mental age was that of a child and he expressed himself as such. Due to the years he spent in a comatose / autistic state, David did not mature beyond the 10-year-old level he had reached prior to the trauma of the Palestinian attack that catalyzed his powers and fragmented his mind. Beginning from the time he awakened from his catatonic state, David's only "power" was the ability to summon the personalities that represented his three remaining active powers. This unnaturally young version of David Haller represented his core personality up until the events presented in the Age of X.
During this period, David's mindscape reflected the warring nature of his personalities. The landscape was a blend of Paris where his mother was serving as ambassador and Beirut, the home of Jemail Karami whose mind he had completely absorbed at the very moment that his psyche was shattered. Jemail erected a giant obsidian dome to protect David's fragile psyche while he made efforts to reintegrate David's hundreds of splinter personalities. In addition to this defensive zone, there were also monstrous shock-troopers who helped defend the dome from Jack Wayne's efforts to seize control.
After a second extended catatonic state, Legion's mind adapted itself in the events leading up to the Legion Quest, creating a version of himself that could access his Omega Level potential. Legion, the God-Mutant was originally presented as a fully merged David Haller, with all of the independent personalities fully integrated into his consciousness, allowing him to access all of his powers. The God-Mutant was later revealed as a distinct, separate persona and has recently been shown simultaneously in the mindscape along with David's original personality. As it turns out, the God-Mutant was only another one of David's personalities, in this case one that apparently represented his ideal image of himself if he were healthy. Of course, this "ideal image" was one crafted by a severely mentally imbalanced young man. The result was similar to the "Merged Hulk" believed to be the ultimate expression of Bruce Banner's consciousness with all his personalities merged into a single psyche. Later on, this Merged Hulk started appearing in Banner's mindscape alongside his other incarnations making it clear that he was just another splinter personality, Banner's idealized version of what he could or should be. "Legion, the God-Mutant" is likely a similar concept expressed by David Haller's mind.
After the Age of Apocalypse, Legion's mind was shattered again by his own psychic knife, once more creating countless personalities. "David Haller" was still childlike, and much weaker than before now that his mind had been split over and over again. Both he and the God-Mutant retreated from the surface of his mind, allowing the myriad of new splinter personalities to run rampant through his mindscape. The mindscape itself took on a new and wilder appearance and now reflected the many different influences and experiences Legion had experienced in his travels through time and space.
Eventually, David reasserted himself and employed a psychological "construct" to control his personalities. After one of David's more vicious personalities gained control of his body and murdered his playmate Marci Sabol, David created a doll he called "Moira", which granted control of his body to whichever personality possessed it. Naturally, the doll, and the control of David's body that came with it, became a coveted item within Legion's psyche.
In addition to the thousands of splinter personalities created by psychic trauma, David also has the ability to actively absorb the psyches of other people who die in his immediate area. These are whole personalities who are integrated into the landscape of his psyche, often receiving one of his splinter powers in the process. Jemail Karami, Marci Sobol, and the Age of Apocalypse's Lucas Bishop are known victims of this phenomena, and it was alleged that numerous alien lifeforms Legion encountered in his recent travels also died and were absorbed into his psyche.
THE NEMESIS PROTOCOLS
Subsequently, Dr. Nemesis and the X-Club developed the Nemesis Protocols. This technique involved transforming the landscape of David's mind into an environment where he felt he had greater control of his personalities. This included the cataloguing and confining of his personas. Nemesis envisioned this in terms of containment cells which together looked much like a concentration camp. His intention was to provide David with a clear metaphorical representation of the isolation and control he was seeking to craft within David's psyche.
David could now access his personalities and their powers at will. He used a command helmet designed by the X-Club as an interface, creating a virtual brig where David had the power to "parole" his personas. He could summon a given personality's cell, allow them access to the physical world where they could use their abilities for his benefit, and then return them to their isolation block on command.
As part of his psychic landscaping, Nemesis took his work a step further and made efforts to destroy some of David's personalities. Nemesis's theory involved the conservation of mental energy. Personalities had been killed or even cannibalized by other personalities in Legion's mindscape in the past, but their powers and essence were not "lost". Instead, they were restored to the greater pool of mental energy found inside David's mind. Nemesis planned to enact an extreme version of Jemail Karami's original efforts by killing personas to restore their "energy" to David's core consciousness. This more radical action against David's psyche resulted in the creation of the Moira persona, a subconscious rejection of Nemesis's work, which acted as a psionic "anti-body" to protect the integrity of David's mind.
During her Age of X, the Moira personality altered David Haller's core personality to create Legion the Force Warrior. There's been no sign of the unnaturally young David since the Age of X, which suggests that Legion retains the memories and experiences he gained from the AoX. This means "young David Haller" is still the dominant personality, only now he has an artificial lifetime of experiences and maturity to fill in for the actual development he missed out on while in a coma. This makes him, for all intents and purposes, an adult for the first time in his life.
This more mature personality, combined with the device that gives him access to his full range of powers, has allowed David to function in the world in a way he has never been able to before. At Professor Xavier's insistence, Nemesis has refined the technology allowing Legion to access his powers via a wristband. Psychologically, the focus is now on cooperation in David's mind, not isolation and control.
The wristband allows Legion to summon the powers of one of his personalities by keying in their catalogue number on the pad. He can maintain these powers for a short period of time without relinquishing control of his body to this personality. However, the longer he holds onto a given ability, the stronger the personality's influence becomes over his actions.
I RULE ME
For some reason, David returned to the prison metaphor to control his personalities while residing at the Tibetan monestary. This Qortex Complex returned David's personalities to a series of prison cells, under constant guard by the Hazeguard and the Xtractor. The Hazeguard was David's mental representation of his shaman, Merzah, who could also actively inhabit the Hazeguard construct through their mental link. The Xtractor was David's own inner self. The Hazeguard helped David bring the various personalities for "Xtraction", where the Xtractor drained his personas to access their powers before locking them up again.
The Complex degenerated into an ongoing prison riot after David learned of his father's death. He spent most of his time in his inner mindscape hiding from the more volatile personalities while occasionally jumping personas and using his remaining Xtractor glove to drain them into submission and access their powers. As time passed, David learned that self-confidence and having a specific goal helped focus his mind, giving him the power to dominate his personalities more easily and eventually causing them to work with him without coersion.
Eventually, David's mental progression led him to accept his other selves instead of dominating them. His mental landscape reshaped from a prison into a psychedelic landscape. The mental David merged with his personalities, assimilating into a larger, more powerful, and more cohesive whole. The gestalt David was able to access all the powers of his absorbed personas, but his mental balance was thrown off by the malignant Beast Xavier personality being absorbed before he was ready. David lost control of his mind-absorbing talent in the real world, creating the Worldworm which was powered by the forcibly assimilated consciousnesses of the entire mutant race.
Inside, David's mindscape was literally melting as the gestalt David trudged through the chaos separate from only the two most powerful remaining personas, the Chronodon and the Weaver, who represented power over time and reality. A psychic message from his father relayed by Blindfold gave him the self-confidence needed to merged with the Chronodon, but nothing would stop the Worldworm.
The Weaver had always been a mysterious figure in David's mind, a colossal spider towering high into the clouds, keeper of David's power to manipulate reality. In his final moments, the gestalt David finally interacted with the Weaver, and discovered the spider actually had his face and voice. The Weaver called David a splinter personality before the two finally merged and ended the Worldworm. One interpretation is that the Weaver had been the "real" David all along, and the Legion we followed over the course of X-Men Legacy was just another splinter personality who believed it was the "real" David. More likely, however, is the interpretation that there is no "real" David. Throughout his series, Legion had tried to dominate and control his "lesser" personalities through various ways, but the final revelation was that all of David's personalities were fragments of the true whole, even the David who self-identified as "me". He was a splinter, because they ALL were splinters. It was finally accepting that fact, that reality, which allowed him to merge with the Weaver.