WARNING: The following article contains spoilers for Way of X #3 by Si Spurrier, Bob Quinn, Java Tartaglia, VC's Clayton Cowles, and Tom Muller, available now.
In his attempt to halt Onslaught's return, Nightcrawler has already allied himself with Omega-Level mutant Legion, splitting his time between the Onslaught crisis and establishing a mutant religion. Legion, on the other hand, is solely focused on hunting his father, Professor Xavier's, dark side.
As powerful as Legion is, even he isn't a match for Onslaught on his own. Luckily, he's enlisted some serious back-up in the form of two immensely powerful mutants with a convoluted history: the Xorn Twins.
Xorn first appeared in New X-Men Annual 2001, by Grant Morrison and Leniil Francis Yu, and was later revealed to be Magneto in disguise. Shen Xorn was then reintroduced as a separate character in X-Men #157 by Chuck Austen and Salvador Larroca. The Magneto who appeared in New X-Men was retconned to be Shen's brother, Kuan-Yin Xorn. He impersonated Magneto under the influence of the sentient bacteria Sublime.
The Xorn Twins are first seen accompanying Legion in hovering above a party on the newly terraformed Mars. Their observation of the scene is interrupted by Pixie, a young mutant Nightcrawler has taken under his wing. She asks Legion to help her friends, Mercury and Loa, with their relationship problems.
Loa's ability to shred things makes her attempt to be intimate with Mercury difficult. Pixie explains that Loa has trouble controlling her powers when she's "excited," causing Mercury to lose one of her liquid metal hands. Pixie asks Legion to help Loa and Mercury with a technique he used to be intimate with the precog Blindfold.
Legion melded Loa and Mercury's minds, allowing them to have the equivalent of a million dates in a moment. It allowed the young couple to share everything with each other. Unfortunately for Mercury, that included a series of dark secrets that escalated into violent fantasies.
It turned out that Onslaught was egging Mercury on from inside the mind meld. Onslaught departed after Legion and Pixie discovered his presence by heading into Loa and Mercury's minds. Pixie then chastised Legion for using her friends' "awkward psychic hookup" as bait for Onslaught. Legion reasoned that it was better that they get it out of the way before they were in a committed relationship.
The pair, along with the Xorn Twins, tracked Onslaught back to Krakoa, where the entity was manipulating the usually peaceful mutant Lost into attacking former Acolyte Fabian Cortez, who was making a scene in Stacy X's mutant brothel.
The battle threatened to collapse the roof of a mutant nursery in the brothel until the cavalry arrived. The sight of Legion, Pixie, and the Xorn Twins was enough to make Onslaught flee once again.
If Onslaught is afraid to confront the Xorns and Legion head on, it is for good reason. Their powers come from literal stars contained in their heads, a sun for Kuan-Yin and a black hole for Shen. While Shen is primarily known as a healer, he and Kuan-Yin have offensive powers as well that includes teleportation, gravity and magnetic manipulation, and the ability to mimic each other's stars.
Shen showed how dangerous he could be when he defeated a group of Apocalypse's Dark Riders by himself. Kuan-Yin has shown the kind of damage he could wreak in horrible ways. While posing as Magneto, he leveled New York City, killing thousands. He one-upped that in Jonathan Hickman and R.B. Silva's Powers of X #3 as a Horseman of Apocalypse. He used the power of the singularity in his brain to end his entire timeline. That kind of power, combined with Legion's, is enough to give even a villain as deadly as Onslaught reason to pause.