WARNING: The following contains spoilers for X of Swords: Creation by Jonathan Hickman, Tini Howard, Pepe Larraz, Marte Gracia, & VC's Clayton Cowles, on sale now.
The "X of Swords" crossover kicked off in X of Swords: Creation #1. Apocalypse has been at the forefront of the build-up to the confrontation between The First Horsemen and Saturnyne and remains a central figure in the first part of the story. After a tense meeting with the Quiet Council over the creation of the External Gate, Apocalypse leads a team of volunteers to Otherworld. Aside from Rockslide, they all share something in common; they're a veritable all-star team of former X-Factor members.
Apocalypse's volunteer force includes members from different eras of the team. Founding X-Factor members Archangel and Beast are the most prominent members of the expedition, and the ones with the deepest ties to Apocalypse. The original X-Factor were the first X-Men to encounter Apocalypse. The ancient mutant's first appearance was in Bob Layton and Jackson Guice's X-Factor #5 in 1986. Before he became an antagonist to the wider X-Men universe, he was a recurring thorn in X-Factor's side. His history with one member of the team is especially bitter.
In spite of Apocalypse's integration into Krakoa, Archangel still doesn't trust him. He isn't going to give the benefit of the doubt to the man who transformed him into the Angel of Death back in his X-Factor days. He understandably wants to keep an eye on what Apocalypse is up to. Beast tells Archangel that his interest is pure curiosity. Given his recent actions, it's hard to trust he wouldn't do something to make any situation he's in worse. Luckily for the X-Men, he doesn't have time to come with a plan more complicated than trying to get a wounded Apocalypse back to Krakoa.
The representatives from the 2nd generation of X-Factor, the government sponsored team, are old lovers Havok and Polaris. They've been apart on Krakoa. Havok has been busy with the Hellions and another old flame, Madelyne Pryor. Polaris has had her hands full with the Kraokoan incarnation of X-Factor. Havok and Polaris are long tenured X-Men. They also have impressive power sets. Despite that, they're both serving as surrogates for more famous members of their families on Apocalypse's mission. With the mission not being sanctioned by the Quiet Council, Cyclops asks Havok to go in his place. For her part, Polaris is representing her father Magneto and the House of M. They share a brief chat about how often being a citizen of Krakoa involves blindly charging through a portal into danger, before doing exactly that.
Three X-Factor Investigations alumni are also part of the team. Siryn has a personal reason for taking part; her father Banshee was injured on a reconnaissance mission to Otherworld. Her old frenemy on the team, Monet, is curious about Otherworld. Having never been, she wanted to see it for herself, in case she decided to take a job running it in the future. While that sounds arrogant, she's enough of an overachiever to do it. The third member of the team has the biggest connection to Apocalypse. As Archangel points out, Rictor has been inseparable from him lately. As seen in the pages of Excalibur, Apocalypse has taken him under his wing, helping him control the seismic powers that kept him from joining the rest of the mutants on Krakoa.
Apocalypse's interest in helping Rictor isn't altruistic. He's proven integral to Apocalypse's plans. An overpowered Rictor used his powers to destroy three Externals, which gave Apocalypse the energy he needed to create the External Gate.
With Apocalypse sidelined, Polaris, Monet, and Havok play a key role in the story. They allow the rest of the team to retreat with their wounded by holding off the First Horsemen. After Monet calling her out doesn't work, Polaris literally drags Saturnyne and her floating ivory tower into the fray. This sets the series' plot in motion, with the titular mutant sword-bearers set for trial by combat against the Horsemen. While they aren't among the sword-bearers, the trio that stayed behind in Otherworld look like they'll still be important to the story.