This article is from the X-Men Comic Books FAQ, by Kate the Short (email@example.com) with numerous contributions by others.
The Twelve, in no particular order, are Magneto, Xavier, Cyclops, Cable,
Jean Grey/Phoenix, Mikhail Rasputin, Iceman, Storm, Sunfire, Polaris,
Bishop, and the Living Monolith/Living Pharaoh. They were gathered by
Apocalpyse in his quest to become a godlike being with more power than
the Phoenix force or the Celestials.
According to Uncanny X-Men #377 (specifically Apocalpyse):
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 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 the 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.
Nate Grey was the 13th member of this merry band, and was slated to
become Apocalypse's new host body. Unfortunately, things didn't go as
planned. Instead, Cyclops sacrificed himself and became a merged half-
Apocalypse being. The team members consider him dead. This FAQ should
also note that Wolverine served as Death, the fourth horseman of
Apocalypse, during much of the shindig. The being they thought was
Wolverine was actually a Super-Skrull so brainwashed into his disguise
that even *he* didn't know he was anything other than Wolverine.
Of course, the final version of the Twelve contradicted much known about
the Twelve up to that point. First, the original Twelve were a vision by
a deranged Sentinel of the dozen most important mutants. The problem is,
these twelve had shown up in other issues (Power Pack #36, X-Factor #14,
X-Factor #68) as well, and even they hadn't been consistent.
The members who had shown up in every version of the Twelve were Xavier,
Cyclops, Jean Grey, Storm, and Franklin Richards. What of the other
seven? In one version, all the original X-Men were implied. Cable, in
both baby and adult forms, was shown in another. Cannonball was clearly
shown in one version. So, we add Iceman, Beast, Archangel, Cable, and
Cannonball to the list. Other possibilities included Dani Moonstar,
Mystique, Psylocke, Wolverine, and Dark Beast.
Uncanny X-Men #-1 cleared up the origin of The Twelve, albeit somewhat
ambiguously. Sometime after Rachel Summers switched places with Captain
Britain in the timestream (Excalibur #75) she rescued and befrended
another time-lost refugee from the 20th century named Tanya Trask, the
daugter of the original creator of the Sentinels, Bolivar Trask. Upon
learning of her father's genocidal legacy, Tanya determined to alter the
timeline by reaching back to the past and convince her father of his
doomed path. Rachel followed her back and erased the memory of her
contact with her father, seemingly setting the timeline right again.
However, it turned out that Tanya's true plan was to store within the
memory banks of Trask's first Master Mold--so deeply even Trask himself
(and later his son and successor Larry) were unaware of it--the identity
of the infamous "XII", The Twelve, whose failure Tanya determined as the
true cause of her dark future and without whom, somehow, mutant genocide
and Sentinel conquest will be averted. Their identity, however, was