M'Kraan Crystal

In the fictional Marvel Comics Universe, the M'Kraan Crystal (pronounced "EM-kron", or "MA-craan" as in Marvel: Ultimate Alliance the video game) is a gigantic crystalline artifact that lies at the nexus of all realities. The Crystal contains a negative galaxy, a source of near limitless power, which pulls and traps all matter inside. The Crystal plays a prominent role in several major storylines, including the Dark Phoenix Saga and the Age of Apocalypse. The Saga states that the Phoenix Force is the legendary guardian of the crystal.

Abilities
As the Nexus of all Realities, the Crystal can allow travel between universes, but opening the Crystal could cause the mass of the neutron galaxy inside to suck in the entire universe and cause a chain-reaction across the Omniverse, destroying all universes in existence. The city inside can only be reached once every million years: when specific galaxies are aligned, the M'Kraan Crystal will allow entry for a short period of time. It also houses the White Hot Room, the place in which the Phoenix Force places all beings to heal them or return them to life. As it is a singleton across all universes, the reality-hopping Exiles have recently figured out how to use the Crystal to visit selected realities; on the Age of Apocalypse, Dark Beast discovered a means to travel through the Crystal to alternate Earths. The Exiles used it as a means to reach the Panoptichron, a place that exists outside of time and space. It apparently can also repower depowered mutants, as Professor Xavier regained his powers after being trapped in and escaping the M'Kraan Crystal (Rise and Fall of the Shi'Ar Empire).

History
The M'Kraan Crystal first appeared in Uncanny X-Men #107. Placed on an abandoned planet, known only as The World, the Crystal was considered sacred by the alien race known as the Shi'ar, a symbol of their faith. The mad Shi'ar emperor D'Ken wanted to use its legendary power for his own ends, and so D'Ken's sister Lilandra Neramani allied herself with the X-Men to stop her brother's mad quest, knowing that the Crystal was nicknamed "The End of All That Is". The Phoenix Force, still weakened and inexperienced in using the new host body of Jean Grey, flew to the crystal to try to stop D'Ken, but was sucked in with everyone else. Inside they found a large endless city, at the center of which was the heart of the Crystal - a sphere which contained a Neutron Galaxy. The X-Men, along with D'Ken, were rendered immobile by a ray coming out of the orb which caused everyone to live out their greatest nightmares. Only Phoenix/Jean Grey was unaffected. Using the empathic link she had with the crystal, Phoenix/Jean Grey discovered an energy field in the form of a latticework which acted as a living shield for the Neutron Galaxy. The latticework was deteriorating. If it failed, the intense gravity from the Neutron Galaxy would cause all of creation to be sucked into it and the resulting Big Bang effect would give rise to a new virgin universe. Phoenix/Jean Grey used her powers to try to repair the latticework but found she wasn’t strong enough, so she reached out telepathically to her fellow X-Men for aid, and with their combined power Phoenix/Jean Grey was able to repair the Crystal and thus save the universe from ultimate destruction.

The M'Kraan Crystal is also sensitive to disruptions of history and reality. When Charles Xavier's son, Legion traveled back in time to kill Magneto, he instead killed Charles. This resulted in a disruption within the Crystal which exploded with a massive wave of energy which washed over the universe, crystallizing everything that existed, before destroying it all. This resulted in a time being 'restarted' as the Age of Apocalypse. During the AoA the M'Kraan Crystal was a major factor as it was discovered that a small shard of the M'Kraan Crystal contained the power of the full Crystal. Apocalypse was planning on using the Crystal to conquer other universes, but Bishop used the Crystal to travel back in time and restore reality. A small shard of the Crystal was responsible for moving X-Man and Holocaust from the Age of Apocalypse Earth to Earth-616, while Sugar Man and Dark Beast entered a larger version to make the same trip. Holocaust would have this shard embedded in him for a while, draining power from the Crystal until X-Man removed it.

During the X-Men: Emperor Vulcan limited series it was revealed that the World was once inhabited by the race known as the M'Kraan, until the Shi'ar attacked them, killed a great number of their people, making the rest flee for their lives. Eventually, the Shi'ar settled on their planet, took the M'Kraan Crystal as their own, and passed down the legend of the M'Kraan Crystal as a sacred gift from their deities, Sharra & K'ythri. The M'Kraan, now calling themselves the Scy'ar Tal (translates as "Death to the Shi'ar") returned and tried to reclaim the Crystal as well as to destroy the Shi'ar empire, only to be defeated by Vulcan and the Starjammers. However, at the end of the miniseries it was revealed that the Scy'ar Tal also conquered the World before they settled on it, so who lived there before and built the M'Kraan Crystal remains a mystery.

During the Avengers vs. X-Men storyline, it is revealed that a group of Kree mystics obtained a shard of the crystal and used it to steal a fragment of the Phoenix Force to resurrect Mar-Vell. The Phoenix, however, soon afterwards reclaimed the stolen fragment from Mar-Vell.

Guardians
The M'Kraan Crystal is guarded by Jahf, a small android with tremendous strength and durability. Should Jahf be defeated, the gigantic robot Modt takes its place. Modt is said to be a thousand times stronger and more powerful than Jahf. Modt claimed that should he be defeated an even more powerful guardian would appear, and that an endless number of increasingly powerful guardians were waiting beyond him. So far no non-cosmic being has been able to defeat Modt. These Guardians seem to exist in all different realities simultaneously. Jahf in the Age of Apocalypse had knowledge of the events on Earth-616. This is in the limited series "Gambit and the X-Ternals". He is seen specifically knocking Wolverine around.

Should the Guardians be avoided, a second layer of defense exists: any person entering the M'Kraan crystal will live through his greatest fears. The Phoenix Force generally protects the M'Kraan Crystal against abuse as well; it has also shown the ability to overwhelm other guardians of the crystal with ease.

White Hot Room
The M'Kraan Crystal is the doorway to the Nexus of all Realities (past, present and future) which in fact is the White Hot Room, also known as the center of creation and the heart of the Phoenix, although Jean has stated that the White Hot Room exists on another plane of existence beyond everything. Nevertheless, the White Hot Room can be accessed through the M'Kraan Crystal.

The White Hot Room is a realm made of pure energy (the Phoenix in its native state) and borders all realms from the material to the imaginary. It is from this realm that the various hosts and the avatar of the Phoenix do their Phoenix work, while they wait to resurrect. Entire universes can be absorbed into this plane and healed or destroyed without even a hint of paradox. The White Hot Room also serves as a waystation that all beings pass through at one point in time before their birth or after their death until they reach their destination either in life or the realm of Death; in essence "You are always there waiting for yourself to arrive".

The Tallus
The Tallus is a communications device, used by the Timebreakers to communicate with their groups of dimension-hoppers Exiles and Weapon X. One Tallus was shown to possess a shard of the M'Kraan Crystal, and it is believed that every Tallus that exist has a shard of the M'Kraan Crystal which enables the teams to travel from one dimension to the next.

Television

 * The M'Kraan crystal appears in the 1992 X-Men animated series. As in the comics, it was sought after by D'ken who wanted to harness the power of the crystal to become, in essence, a god. Absorbing the power of the crystal did indeed transform D'ken but also sucked himself, as well as the X-Men, the Starjammers, the Shiar Imperial Guard, and slowly but surely the rest of the universe as well into the crystal. Within the crystal, D'ken seemed capable of almost any feat, from altering his physical form and firing energy blasts, to controlling the matter all around. Eventually, the Phoenix, having apparently mastered the full range of her powers, appeared and rescued the trapped X-Men and Shiar guard while leaving D'ken trapped inside the crystal. The Phoenix then took the Crystal into the heart of the sun, knowing that while it could not be destroyed, it would still be safely hidden within.

Video games

 * The M'Kraan crystal appears in the video game Marvel: Ultimate Alliance. It is used to generate a shield around Deathbird, and a shard of it is used in conjunction with the Muonic Inducer (a device stolen from Galactus) to absorb some of the power stolen from Odin by Doctor Doom so that the heroes can fight him on more equal terms.
 * The M'Kraan crystal also appears as a collectible item in Marvel: Ultimate Alliance 2, broken into shards. Collecting five shards will result in unlocking Jean Grey as a playable character.