Omega-level mutant

Omega-level mutant is a fictional designation for a certain powerful class of individuals in Marvel Comics to indicate mutants with the ability (or potential) to exist beyond the physical realm and consciously explore the mystical. Immortality is also a possibility, but not a requirement. Being able to control matter on the sub-molecular scale also seems to be a requirement.

Omega implies unlimited and ultimate potential. The limited series, X-Men: Forever spoke of how humanity sees the result of experiments the Celestials carried out on early lifeforms, on behalf of the abstracts entities of the Marvel Universe (Eternity, Death, and so forth). The reason for these experiments was to ensure that a race of beings would eventually evolve that would surpass and in turn replace all of the Abstracts. This would bring about the end of the universe. By creating humanity the abstracts are ensuring that the cycle of life continues to revolve. This final state of evolution is Phoenix. It's what all humanity will eventually turn into. That's why Jean as Phoenix is called the "ultimate mutation". Those with omega (the last) potential are the ones closest to reaching this final stage of evolution. Whether their potential results in they themselves turning into Phoenixes (like Jean Grey and Quentin Quire have) or their descendants doing so, within Omega-level mutants lies the potential to bring about the end. Hence its name.

The Dark Phoenix Saga involved an Omega mutant, Jean Grey hosting the Phoenix Force, which was referred to by the Shi'ar as "the nexus of all psionic energy which does, has, and ever will, exist in all realities of the multiverse. During its time as a sentient entity, it was known to possess the psyche of mortals (particularly those who happen to be telepaths) and thus amplify their psychic abilities to a cosmic (and in some cases, nearly incalculable) scale. Its primary form is that of a gigantic firebird." In X-Men: Phoenix: End Song, the Shi'ar attempt to kill both Jean Grey and Quentin Quire as they are omega-level telepaths who might host the Phoenix Force. In Uncanny X-Men #467, the Shi'ar kill most of Jean Grey's relatives in order to prevent another host for the Phoenix.

There are also more classifications of mutants in the Marvel Universe, though only mentioned briefly by Apocalypse, Sentinels, the US Government and during the Age of Apocalypse crossover. They are Alpha (Apocalypse classes himself, his "Horsemen", and a few other especially powerful mutants as Alpha level), Beta and Epsilon-Delta (Joey and Gailyn Bailey, the nephew and niece of Jean Grey, where regarded as Epsilon-Delta by Sentinels during "Operation: Zero Tolerance").

Omega-level mutants

Character	References
Elixir	New Mutants vol. 2 # 12 by Beast Also: New X-men: Academy X #1
X-Men: Forever by Professor X
Jean Grey	X-Men: Forever by Professor X Also: New X-men #128, New X-men #133
Mister M	The 198 Files
Franklin Richards	The 198 Files
Quentin Quire	Phoenix Endsong #2 by various sources
Vulcan	X-Men: Deadly Genesis #1 by Emma Frost
Rachel Summers	Uncanny X-Men #208 by Nimrod Rachel is also a former Phoenix avatar.
Mr. Immortal	By Deathurge. Recovers from any fatal wounds