The parentage of the X-Men's veteran teleporter Nightcrawler was a major mystery for a time, with his mother finally revealed to be the long-time villain Mystique and his father turning out to be the devilish Azazel. But as his family tree expanded, Nightcrawler learned he had siblings as well -- including one who might have the potential to be an even more powerful teleporter.
The mutant Abyss debuted in Age of Apocalypse: Alpha (by Scott Lobdell, Mark Waid, Roger Cruz and Steve Epting) years before he was introduced to the 616 Marvel Universe, and could potentially be a game-changing teleporter for Krakoa. However, this potentially powerful mutant has been missing for quite some time.
Across realities, Abyss is an Alpha-Class Mutant, and a powerful teleporter. He's capable of opening a void to the Brimstone dimension within his chest, allowing him to teleport in a manner similar to Nightcrawler. Abyss also has the ability to suck his enemies into that other dimension. Nils Styger had the distinction of debuting in the Age of Apocalypse reality before his first appearance on Earth-616. In that hellish world, Abyss had risen in the ranks of the mutant villain, even achieving the rank of Horseman before falling in battle against the X-Men. However, on Earth-616, Nils was born in Genosha and grew up living a relatively normal life. Secretly the son of the devious mutant Azazel, Nils spent most of his life unaware that he was the half-brother of Nightcrawler.
Nils was one of the numerous mutants afflicted with the Legacy Virus. He worked to protect the Genoshan scientist Renee Majcomb, who was developing a cure for the disease. Luckily, the sacrifice of Colossus resulted in the Legacy Virus being cured from across the globe. A much more meek but heroic figure than his Age of Apocalypse counterpart, Nils was drawn into the conflict at the heart of "The Draco," wherein Azazel captured all of his various children in an attempt to breach the Brimstone Dimension and return to Earth. Working with the X-Men, Abyss was able to prevent this extradimensional invasion. Abyss was one of the many mutants to lose his powers during the events of M-Day, and briefly joined a terrorist group known as the X-Cell.
However, when he and his allies regained their powers thanks to a Terrigan Mist-infused Quicksilver, they battled X-Factor Investigations. Upon realizing his friends had become unstable in the process, Abyss dragged them into the brimstone dimension in an attempt to contain the overload of energy and save nearby civilian lives. However, he was later captured by AIM and experimented on -- losing access to his powers only to be rescued by the then-current X-Factor team. Abyss was last seen escaping the lab, and hasn't made his presence known in the era of Krakoa.
Still, Abyss might be the perfect candidate for residency on the X-Men's island. He could go through the Crucible and earn the chance to have his powers restored, and his sheer potential as a teleporter makes him an ideal backup candidate for the Six. He could also provide a fun sounding board for Nightcrawler, giving the classic X-Man a blood relation that he doesn't actively have to contend with (unlike his parents). The Age of Apocalypse version of Abyss proved just how powerful he could potentially be, and he'd make an interesting addition to the current X-titles, filling a unique role in the process.