Of all the amazing alternate versions of the Marvel Universe , the Age of Apocalypse just might be the most well-remembered. Long after the storyline that introduced it ended, this dark version of the Marvel Universe is one that has become iconic for its bleak tone and stark contrast to the world that came before it, and that carried through into its ending.
Although the world of Age of Apocalypse seemingly came to a brutal end during 1995's X-Men Omega #1 by Scott Lobdell, Mark Waid and Roger Cruz, it continued on as an alternate reality long after the death of Charles Xavier was averted. Thanks to the intervention of that reality's Phoenix Force, it remained as its own separate universe, and the Age of Apocalypse's version of Nightcrawler went on to work alongside the Marvel Universe's X-Men in Uncanny X-Force.
When the team encountered Wolverine's Uncanny X-Force from the primary Marvel Universe, Nightcrawler joined them in their return home in order to make the Iceman and Dark Beast of his reality suffer for their crimes against life. During his time in the main Marvel Universe, this Nightcrawler betrayed the team that had taken him in by murdering the Blob, and to escape his pursuers he teamed up with Dark Beast. This pairing kicked off the beginning of the end for the Age of Apocalypse in 2013's X-Termination #1 by Marjorie Liu, Greg Pak, and David Lopez. The Hank McCoy of the Age of Apocalypse used the power of the dormant Dreaming Celestial to return home, but that also subjected every world that the Celestial visited to the terrifying Exterminators.
The Exterminators were ancient creations of the Celestials that acted as bringers of death. Their proficiency and proclivity towards their duties left them far more destructive than the Celestials' had intended, so they were sealed away in one of the realities among the multiverse. The portal that had been opened by Dark Beast and Nightcrawler gave the Exterminators a path to the Age of Apocalypse world, where they encountered the X-Men of the primary Marvel timeline, a mutant team from the alternate world called the X-Terminated, and a strange trans-dimensional X-Men team.
Although the different versions of the X-Men were in conflict with one another, they all came together to fight the threat posed by the Exterminators. The mutants were no match for the beings, even after the Jean Grey of the Age of Apocalypse was imbued with the power of Apocalypse himself. As Nightcrawler evacuated survivors back to the primary Marvel Universe, he realized that the black shards coming through the portal were the physical manifestation of the barrier between worlds. That being the case, he came up with the perfect solution to their problem, although it would cost him his life. Using the shards of the barrier as handholds, Nightcrawler teleported the end of the portal on the primary Earth all the way back to his own reality, effectively tying it off like a knot. This drastic measure once again sealed the Exterminators off from the rest of the multiverse along with the universe they then inhabited. This led to the true effective destruction of the Age of Apocalypse reality, cutting it off from the rest of the multiverse.
Despite the loss of an entire universe, the impact that the Age of Apocalypse left on those who had come to know it is still being felt today. There were a few survivors of that world, most notably Nate Grey's X-Man and Blink, who have had semi-regular presences across Marvel's X0Men comics over the years. Dark Beast also survived to cause more trouble, but his machinations met a brutal end at the hands of Magik.
While the Age of Apocalypse may be one of Marvel's most famous worlds, the current state of the Marvel multiverse means that it stands in isolation, despite a legacy that's spread far and wide around the Marvel Multiverse.