First Appearance: X-Men (1st series) #107
Location
The M’Kraan Crystal was located on an empty, unnamed world that was so far away from everything else in the universe that the lights in the sky were not individual stars, but entire galaxies. Shi’ar legends say that this was the first planet to form. While races have lived on the world previously, all traces of these races were long gone and the only structure that withstood the ravages of time was the M’Kraan Crystal. Following its discovery by the Shi’ar, the planet was placed under their control, up until the planet was destroyed in the war with the Scy’ar Tal. After this, the M’Kraan was seemingly relocated to another unnamed word.
Description
The M’Kraan Crystal is an immense, pink crystal, which is older than known history. Inside of the crystal, there is a beautiful, alien city, which outstretches by far the room that should be within the dimensions of the crystal. At the center of the city is a glowing energy sphere, which hosts a neutron galaxy. The sun of this neutron galaxy is a nexus point of all matter and antimatter, of not only the universe but all different realities in existence. This is why the crystal is often referred to as the “Nexus of all Realities.”
Though the M’Kraan functions like a doorway, it is necessary that it is kept closed. This is true not only in one but all realities. Otherwise, matter would cross over from one reality into the next and, sooner or later, the draft would create a cascading effect that would finally erase all timelines. All matter would be pulled into the N-galaxy and, in a Big Bang, a new universe would replace the existing one. For this reason, old legends call the M’Kraan crystal “the end of all that is.”
Guardians
To ensure that nobody enters the crystal and misuses the nexus point, the M‘Kraan has various lines of defense. The first guardian is a purple dwarf named Jahf; although he doesn’t look very tough, he is an almost invincible robot. The second guardian is a mechanical construct called Modt, who can grow to giant size. Finally, if somebody is able to get past them, they most probably will fail inside the crystal, as the energy sphere emits a nightmare field that stops all those who want something of it without offering anything in return.
Interactions with Jahf in the Age of Apocalypse showed that Jahf and Modt are without alternate counterparts, the pair of guardians exists only once and they have to watch over the crystal simultaneously in all different realities.
History
Thirteen thousand years ago, the race known as the M’Kraan discovered "the World," the planet that housed the M’Kraan Crystal (which they either named after themselves or named themselves after). The space around the planet, however, was inhabited by an unidentified race. Believing the planet to be their own holy land, the M’Kraan forced the current inhabitants out and claimed the planet for themselves. The M’Kraan dwelled in orbit around the crystal, worshipping it for twelve thousand years until the Shi’ar discovered the crystal. In a repeat of fate, the Shi’ar expelled the M’Kraan from the homes and claimed the crystal for themselves. [X-Men: Emperor Vulcan #2, 5]
The Shi’ar believed the M’Kraan Crystal was their birthright, that within it is housed the palace of their gods, Sharra and K’ythri. As such, the Shi’ar Empire protected the world fanatically for a millennia.
Years after D’Ken Neramani became Shi’ar emperor, he discovered rumors about an ancient power source called “the end of all that is,” which he learned was in fact the M’Kraan. He thought to explore this power, but his younger sister Lilandra, serving as a Grand Admiral, opposed him, thinking the mission too dangerous. In retaliation, D’Ken framed Lilandra for treason and condemned her to death. Fortunately for her, Lilandr escaped and made her way to Charles Xavier on Earth, having recently formed a psychic report with him. D`Ken contacted Shakari, one of the empire’s agents on Earth, who kidnapped Lilandra and took her back to the M’Kraan world. There, D’Ken planned to execute his sister and seize power from the crystal during a rare galactic alignment.
Now involved, the X-Men followed Lilandra and emerged right in front of the huge M’Kraan Crystal. After a brief but intense battle with the Imperial Guard, the X-Men rescued the princess from certain death in form of a creature called Soul-Drinker. Keeping the X-Men busy with his own Imperial Guard, D`Ken bought himself the time he needed until a group of stars shifted into place to activate the M’Kraan Crystal.
Before anyone present could enter the crystal, the guardian Jahf appeared. At first, Wolverine did not take the purple dwarf seriously but quickly learned otherwise when Jahf’s first punch hurled Wolverine into the planet’s orbit (where he was rescued by the Strajammers’ ship). More impressively, Jahf withstood being crushed by a meteor redirected by Phoenix. When during the course of the battle he mentioned that his programming forbade him from allowing anyone to enter the M’Kraan, Banshee deduced that Jahf was a robot and felt justified to use lethal force to scramble his insides. With the first guard down, Modt, a second, much more powerful guardian, appeared and the battle started anew. While Modt fought the X-Men, Raza of the Starjammers threw D’Ken into the M’Kraan.
With the defense line broken, everyone present was pulled inside, where they found themselves in a beautiful but unsettling alien city. In the center of the metropolis, the X-Men discovered a glowing sphere of energy, which soon activated its own defense mechanism: a nightmare field that projected scenarios of great fear and horror tailored to each of the X-Men. Unaffected by the attack due to her recent transformation, Jean witnessed as one of Scott’s stray beams damaged the energy sphere. Investigating the damage, she discovered that a neutron galaxy existed within and soon realized that if its energy lattice was not repaired it would draw in all of the universe’s mass, resulting in a new Big Bang and creation of a new universe. Though she knew that she could use her energies to heal the sphere, Phoenix feared that she would be fully absorbed as well. To anchor herself to humanity, she linked with the lifeforces of Storm and Corsair right before she flew into the sphere. As she worked, the Phoenix signature effect kept growing and even dwarfed the whole solar system, until finally the impossible task was accomplished and all found themselves outside the M‘Kraan Crystal again. Due to the sights he experienced, D’Ken’s mind snapped and he entered a catatonic state. [Uncanny X-Men (1st series) #107-109]
Not long after his initial experiment with the world’s heroes and villains in his Secret War, the nigh-omnipotent Beyonder decided to come to Earth, where he further investigated the concept of desire and toyed around with its heroes. Eventually growing frustrated and angry with what he found, the Beyonder finally decided to kill the universe, as none of its people could provide him with the sense and purpose he was searching for. Rachel Summers decided that, if the universe should be destroyed, then at least the Beyonder himself should be killed too. At night, she absorbed the lifeforces of her fellow X-Men (even though some of them objected) and departed to space. Arriving at the world of the M’Kraan Crystal, she surprised Jahf before he could even react and then entered the crystal. Inside, she attempted to enter the sphere and destroy it, so that everything, including the Beyonder, would be sucked into the N-galaxy. When Rachel linked with every living being in the universe, it was Storm‘s voice that convinced her that she could not kill millions of people who had no idea what their sacrifice would be for. Convinced she was wrong, Rachel returned to Earth and gave back the X-Men’s lifeforces. The Beyonder was ultimately dealt with in another way. [Uncanny X-Men (1st series) #203]
When Professor Xavier’s illegitimate son David Haller tried to create a better world by travelling 20 years into the past to kill Magneto, a small team of X-Men followed him. Unfortunately, the X-Men’s mission went horribly wrong, when Legion’s blast meant for Magneto hit Xavier instead. The Professor’s death caused ripples in reality, which Jahf recognized as disturbances in the crystal instances after the X-Men entered. After made his report to Lilandra, the situation gained the attention of not only one, but a whole group of Watchers. Nothing could be done, however, as Legion’s actions erased the timeline and replaced it with a new one, the Age of Apocalypse. The end came in the form of a crystalline wave that washed over the universe. In the final hours, the crystal indeed lived up to the claim of being “the end of all that is.” [Uncanny X-Men (1st series) #320-321, X-Men (2nd series) #40-41]
With Xavier dead, the 616 universe was replaced with the “Age of Apocalypse” reality. Bishop, a chronal anomaly himself, was unaffected by the change and, after years of wandering around the transformed world, was able to convince Magneto’s X-Men that this was not how reality was meant to be. At first suspicious, Magneto soon believed the enigmatic stranger and the X-Men came up with a plan. 
While other groups were busy tracking down Illyana Rasputin and Destiny, Gambit’s team, the X-ternals (Lila Cheney, Strong Guy, Sunspot, Jubilee) departed to Shi’ar space to acquire the M’Kraan Crystal.
Since in this reality Xavier died very early, the X-Men’s contact to the Shi’ar was never established and neither Phoenix nor anyone else was there to repair the energy lattice. Ever since, the crystal had been flaring with white hot power, terminating various sectors of Shi’ar space. After facing the Imperial Guard and other Shi’ar troops, the X-ternals made their way into the crystal, where Jahf explained what the crystal was. 
In exchange for his undying love for Rogue, Gambit was granted a piece of the M’Kraan, which due to the crystal’s nature was as powerful as the whole crystal itself. Things seemed now to be going to the X-ternals’ way, as they made it back to Earth, where the crystal started regenerating itself. Unfortunately, their luck took a turn for the worse when it soon fell into the hands of Apocalypse himself.
Soon after, the various X-teams united for a final strike against Apocalypse in his stronghold. During the battle, Destiny, Illyana and Bishop entered the M‘Kraan Crystal. Once within, Destiny’s precognitive powers allowed her to choose the right alternate past, to which little Illyana opened a stepping disk for Bishop to go back in time and prevent Xavier’s death. Though he succeeded and the timeline was restored, the Age of Apocalypse continued on as a new, completely separate reality. In the restored timeline, the crystalline wave that had been approaching the Earth simply washed over Earth’s heroes as if nothing had happened. However, not everything remained unchanged. Sugarman and Dark Beast had secretly entered the crystal too and had crossed over into the main 616 universe, though they had appeared twenty years in the past. Additionally, during the big fight, X-Man had used a shard of the crystal to stab Holocaust’s armor. The energy interaction caused them both to cross over as well, although the two arrived in the present, shortly after the crystalline wave. [X-Men Alpha, Gambit & X-ternals #1-4, X-Men Omega]
Sometime later, Jahf alerted Lilandra that four energies were leeching off the crystal, one of them very large. The Shi’ar investigated and the trails ended in the four AoA survivors, who were all captured. It was discovered that a piece of the AoA crystal shard was still inside Holocaust‘s chest and, after X-Man helped to pull it out and destroy it, all four were purged of the energy trails. Afterward, they were all sent back to Earth.
On another occasion, some Infintes crossed over in the Alaskan snow lands. In their home reality, the AoA scientists had been experimenting with interdimensional travel and only partially succeeded, as the test persons were fused together during transportation. X-Man, with the help of Cyclops and Phoenix, closed the gate, but the incident had already caused disturbances within the M’Kraan. Jahf and Modt examined, but found only Nate as a possible source of the problem. To test him, they posed as a wanderer and his son, who had suffered an accident. The son was dying and it was tempting for Nate to try to resurrect him, but he didn’t. Before he made a final decision, he saw through the deception and a battle erupted. Jahf wanted to kill the possible threat to the M’Kraan but Modt objected, as X-Man had shown he was responsible by hesitating and thinking instead of simply resurrecting a dead person. [X-Man Annual '97, X-Man #55]
After so long in a mindless state, D’Ken was finally awoken from his catatonia by the mutant Vulcan. In reward for this, the emperor promised to marry Vulcan to his sister Deathbird at the site of the M’Kraan Crystal, an occasion which would then be followed by the execution of Charles Xavier. It was Gladiator who sought permission to host the wedding there from Jahf, who surprisingly agreed. Like D’Ken, however, Vulcan had his own history with the X-Men, who along with the Starjammers and Shi’ar rebels attacked immediately after the wedding to get at Vulcan. During the fight, Xavier was hurled by Vulcan into the M’Kraan Crystal, wherein he immediately found himself tormented by its defensive nightmare field. Inexplicably, however, the crystal’s sphere simultaneously restored his lost telepathic abilities. Still, Xavier would have perished if not for the X-Man Darwin, who leapt into the crystal after Xavier and saved him. [Uncanny X-Men (1st series) #484-486]
When the M’Kraan species returned from a thousand years of exile, they sought to recover the crystal for themselves. Renaming themselves the Scy’ar Tal, meaning “death to the Shi’ar,” the species developed a devastating weapon able to teleport stars. When the Shi’ar stole this weapon for themselves, Vulcan, who had become since emperor to the Shi’ar, dropped a star on the Scy’ar Tal fleet. The attack destroyed the Scy’ar Tal fleet but M’Kraan and its world appeared unharmed. [X-Men: Emperor Vulcan #1-5]
After the events of the War of Kings between the Shi’ar and the Kree, a massive tear in space and time known as the Fault was created. Gladiator sent a squadron of Imperial Guardsmen into the Fault to investigate, but soon became overwhelmed by a horde of holy horrors. Oracle worked with the cosmic hero Quasar to transmit a telepathic warning through the M’Kraan Crystal to Gladiator, warning him of what was coming. Not content with leaving his guardsmen to die, Gladiator sought permission from Jahf to allow the remaining Imperial Guard to access the nexus of realities within the crystal to teleport directly into the Fault. Due to the extraordinary circumstances, Jahf agreed.
The M’Kraan in Other Realities
As the Nexus of Realities, every M’Kraan in every reality is linked. Miraculously, despite some close calls with the crystal, the multiverse has yet to be destroyed.
One reality demonstrated that it doesn’t necessarily have to be Phoenix who repairs the crystal. In a reality where Jean was never approached by the Phoenix Force, it was Cyclops who first touched the energy sphere and entered the intangible state. He then used his optic blasts to heal the energy lattice by letting his beams flow into it. At the same time, he was constantly boosted by Havok and had the psychic support of the rest of the team. [What if (2nd series) #23]
In another reality, Vulcan fell in to the M’Kraan and entered the White Hot Room. There he met numerous Phoenix hosts from various realities and killed them, stealing their Phoenix power. He left the crystal and wreaked havoc for a time until he was stopped. [What If...? Uncanny X-Men Rise and Fall of the Shi’ar Empire #1]
The Age of Apocalypse reality managed to survive as its own universe after the 616 timeline was restored. The M’Kraan Crystal shard Gambit had recovered fell into the hands of the newly established U.S. government, which studied it closely. When Holocaust was recruited unwillingly into the reality hopping Exiles, he immediately concocted a plan to break free of the mysterious Timebroker who was pulling their strings. As they had been sent to his home reality on a mission, Holocaust seized control of the M’Kraan shard and then recovered Hank McCoy’s notes on the object. Using the shard, the group managed to teleported into the crystal, much like McCoy had done to escape to the 616 a year earlier, and reached the Timebroker’s base, outside time and space. [Exiles (1st series) #60-61]