WARNINGS: The following contains spoilers for Way of X #2 by Si Spurrier, Bob Quinn, Java Tartaglia & VC's Clayton Cowles, on sale now.
In Way of X, Nightcrawler has been trying to isolate what's causing a fatalistic and self-destructive feeling across the mutant population. Learning of something called the Patchwork Man and being recruited by Xavier to solve the crisis, Nightcrawler's first suspect is Legion. But on top of unexpectedly gaining Legion as an ally to his mission, Nightcrawler also learns about the true psionic threat to Krakoa. It appears that Onslaught has been restored in some manner and unleashed against the island nation by Orchis, who hope to sew the seeds of discord in Krakoa now so that they eventually destroy themselves. One of Marvel's most dangerous mutant-born threats, Onslaught is the direct result of Charles Xavier and Magneto's dark sides fusing into a new form, which took place during a classic X-Men storyline.
Onslaught was effectively born out of the events of "Fatal Attractions," a major X-Men crossover that played out across X-Men, Uncanny X-Men, Wolverine, X-Force, X-Factor and Excalibur in 1993. The story focused on the return of Magneto and his Acolytes, announcing their intention to wipe out humanity once and for all so that mutants could take control of the world. Utilizing a Shi'Ar exoskeleton, Professor Xavier is able to personally lead an assault against Avalon by the X-Men to prevent his plans. As the fight raged, Magneto lashed out at Wolverine and ripped the adamantium out of the long-living mutant, almost killing him. Enraged and finally exhausted with Magneto's actions over the years, Xavier used his powers to effectively mindwipe Magneto, leaving him comatose and absorbing some of his psyche.
The process resulted in the creation of a new sentient psionic entity that festered within the confines of Xavier's mind -- growing stronger as the injustices against mutants continued to stack up. Initially personified by Magneto in issues like Uncanny X-Men #309 by Scott Lobdell and John Romita Jr., this element eventually learned how to manifest in the material world thanks to an encounter with Nate Grey -- resulting in Onslaught breaking loose, briefly taking control of Xavier himself and setting off the events of the Onslaught Saga that almost resulted in the deaths of the Avengers and the Fantastic Four. As a result of its creation, Onslaught is a particularly ambitious and dangerous being.
Thanks to the sheer psionic power of Xavier, Onslaught is a strong enough force that it takes entire teams to even draw his attention as well as the full might of the Hulk to even hurt him. Onslaught's return comes at an interesting point not just in the history of the mutant race, but in the relationship of Xavier and Magneto. Both men have been trying to set aside their past differences for the betterment of Krakoa, openly serving on the nation's ruling body (the Quiet Council) while orchestrating things from behind the scenes with Moira X.
Although their unity has been tested in part by questions of how their laws truly apply, it's rare that the pair have been so unified in purpose and intent. The remnants of Onslaught might see this as reason enough to be utilized against Krakoa. While Nightcrawler and Legion are set to deal with the threat directly in the pages of Way of X, it'll be interesting to see how the psionic being's fathers are able to respond to its return -- especially if it helps bring about the premature and chaotic end to their new dream for a mutant homeland.