History: The Twelve were a group of powerful Mutants with legend speaking that they would be responsible for bringing about the demise of Apocalypse. The legend surrounding these twelve beings would survive even into the future where it was said that they would battle En Sabah Nur in the lands of Egypt. Knowledge of the Twelve came to the present when time displaced Tanya Trask attempted to warn her father, Bolivar Trask, in the present by journeying back in time to three years before the formation of the X-Men. However, she was stopped by Mother Askani herself but not before data of the Twelve was uploaded into Master Mold. In the databanks of the Master Mold, this group was identified as a dozen mutant humans who will one day rise up and lead all of mutantkind in war against homo sapiens in the twilight of earth. According to Cable, the Twelve battled Apocalypse and were defeated thus allowing him to conquer the world. Further details of the prophecy surrounding the Twelve were detailed in the Destiny Diaries. This prophecy stated that:
"The Twelve Will Gather in the Darkest Hour
The flame of the rising sun.
A cold soldier returned to his family.
A goddess recently returned from the stars.
A father of many futures connected to a greater evil.
A childless mother who rose from the deep.
A son of a world that may never be, spawned by a sinister plan.
A man whose mind is his greatest ally and greatest enemy.
The ruler of a world gone mad, now free of demons.
A man out of time with no real future.
A North Pole ravaged by a failing spirit, lonely half of a greater whole.
A South Pole, weaker than believed and the heart of an empire.
A ravaged planet, worshipped by the sands of time.
A thirteenth will fall from the sky… The Chosen will know the scars of Apocalypse"
The Master Mold Sentinel would later detect the presence of Cyclops in Alaska and attack him as it identified him as a member of the Twelve. These would be Mutants around whom others would gather whilst they would lead who would shape the way of things in the future. Master Mold survived the encounter and later faced Power Pack in an attempt to kill Franklin Richards. As the youngsters fought the demented robot, he revealed the nature of the Twelve: "The dozen mutant humans who will one day rise up and lead all of mutantkind in war against Homo sapiens in the twilight of Earth." He was stopped, but not before revealing several other possible candidates, including Danielle Moonstar, Cannonball and Psylocke. The subsequent mention of the Twelve came from Apocalypse himself, claiming that not only was he one of the "fabled Twelve", but so too were all five original X-Men, Charles Xavier, Storm, Cannonball and Cable.
As the new millennium approached, a series of events led to the return of Apocalypse. Cable was witness to said events — the darkening of New York City, the arrival of the Harbinger of Apocalypse, and several other signs. Mother Askani's astral form revealed herself to Cable, implanting the knowledge of who the Twelve were. Sadly, he never had the opportunity to find them. Apocalypse began his attack by planting a fake Wolverine among the ranks of the X-Men. This led Xavier, who suspected the infiltration, to disband the X-Men. The X-Men parted ways, and soon after, Rogue and Shadowcat found themselves protecting Mystique from Japan's military, the Yakiba. As Rogue fought Sunfire and his men, Kitty watched over Mystique and found, unwillingly, Destiny's diary. The diary itself had a very cryptic description of a "thirteenth" and told that the Twelve would be involved in the destruction of the world. With the death of Wolverine's imposter, the X-Men reunited at the mansion, only to discover it was not truly Wolverine who had died, but a Skrull. The infiltrator had been found. To save Polaris from abduction, Cyclops took her place using an image inducer, and the X-Men followed him to the Skrulls' lair, where they were attacked by Death, Apocalypse's most lethal and fiercest Horseman, the same person responsible for the death of the fake Wolverine. After a heavy battle, Death was unmasked by Colossus. The X-Men were surprised to see their newest enemy was in truth the real Wolverine. Death escaped, leaving the X-Men with half-truths and enigmas that needed solving. Soon after, Xavier revealed the "List of Twelve", written in Destiny's Diary. The Twelve were: Magneto, Polaris, Storm, Sunfire ,Iceman, Cyclops, Phoenix, Cable, Bishop, Mikhail Rasputin, Professor X, and Living Monolith
The End of The World As We Know It: In reality, the legend of the Twelve of Power was actually created by none other than En Sabah Nur himself as part of his attempt in gaining god-like power. Long ago, he had discovered the inactive Celestial Circuit but lacked the means of powering it. Thus, he created the legend of the Twelve who were all primal alpha-level Mutants that would bring about his ascension to omnipotence above that of the Celestials. Following the defeat of Onslaught, he began his plans and was responsible for the capture of the Twelve who had all unwittingly brought about the fulfilment of the prophecy.
"The Monolith is at the core of it all, as the primal Earth which is the foundation for all that is to be mine. Magneto and Polaris are opposing magnetic poles, serving to control the flow of the energies at play here... Energies under constant pressure from the forces of nature itself. The elemental extremes of Iceman, Storm and Sunfire... stimulated by the unrestrained energies of man and the heavens, free of any grounding or gravity. Father, mother, and son -- Cyclops, Phoenix, and Cable -- far, far stronger as a whole than a sum of their parts... linked to the powers of time and space, wielded by Bishop and Rasputin, respectively. The power of pure thought that is Xavier. As above so below. As without, so within. And as you die, one by one... I shall rise as the sun, at the heart of this new universe. All your power will flow into me -- through me -- through the new Ra, ultimately, the be-all and end-all to come!.. And you, young Nate -- and the unequaled unlimited forces at your command-- play the most crucial role of all in my final design...!" --Apocalypse
The Horsemen of Apocalypse kidnapped Cable, Mikhail Rasputin, Iceman, Sunfire and the Living Monolith. The X-Men, with Magneto's help, rushed to Egypt, to Apocalypse's lair. They were soon attacked by an army of Skrulls and followers of Apocalypse. Amidst the battle, Bishop, who had been trapped in an alternate future reality, appeared out of thin air, furthering the confusion. Thanks to illusions and lies, the agents of Apocalypse were able to kidnap all remaining members of the Twelve. Placing the Twelve inside containment cells, part of an ages-old Celestial machine, Apocalypse revealed his plan was to siphon the energies of the "Eleven of Power" through the Living Monolith to start the machine, which would grant him omnipotence. However, Apocalypse knew his frail, centuries-old body would not be able to withstand such power. And that's when he revealed the thirteenth mutant, the X-Man, Nate Grey. He intended to transfer his consciousness into Nate's young body and then use the energies of the Twelve to evolve. He began the process, but Magneto's then-recent power loss was something Apocalypse had not counted, and soon his energies were depleted, creating a break in the chain of power. Apocalypse increased the flow of energies, attempting to bypass the break, but this drove the Living Monolith insane, destroying his containment unit and starting a rampage that unwillingly released the X-Men. While half of the team battled the Monolith, the other half attempted to stop Apocalypse. Jean Grey exposed the true nature of Apocalypse, showing his rotting body inside his armor. As he tried to enter Nate Grey's body, Cyclops took his place, sacrificing himself to save Nate. Jean attempted to contact her husband's mind, but Xavier claimed there was nothing of Cyclops left inside Apocalypse.
The story carries on into the Ages of Apocalypse arc, where Apocalypse using the energies gathered from the Twelve, re-made reality twice, attempting to re-create the process once more and finish his transformation. He was unable to do so, for the X-Men fought him both times, in the past and the future. His energies spent, he and the Living Monolith escaped. The X-Men had prevented Apocalypse from becoming a god, at the seeming expense of Cyclops' life. Heartbroken over her husband's death, Jean took a leave of absence from the X-Men.
Others members who were thought to be part of this gathering: Master Mold considered potentially as part of the Twelve Franklin Richards. Kitty Pryde believed that members of the original five Angel and Beast (especially this former because of his bound to Apocalypse) could be part of the Twelve, as well as Wolverine, seeing his lifespan as a sign of his role to be played. She also initially believed that Alpha-Level Mutants (who included the Twelve Phoenix, Professor X and Storm) could be part of them but started disregards her own theory learning that Sinister was part of the list, and eventually thought it to be pointless.
For more indepth breakdown read: THE TWELVE - PLOT AND UTCOME by Peter Luzifer.