Things on Muir Island had been odd for some time. Polaris was exhibiting odd powers, Moira hadn't acted like herself for months, and the Muir Island X-Men attacked known allies during the Kings of Pain arc. The catalyst behind all the oddities: the Shadow King, a being intent on destroying Xavier's dream. Taking the host of FBI Agent Reisz, the Shadow King orchestrates his schemes in both the physical and psychic realms, a feat possible by using Polaris as his nexus between the two worlds. Having grown stronger, the Shadow King emits negative feelings, an effect felt across the globe, as people everywhere suddenly begin acting on their hatred and prejudices.
The mutants on Muir Island have been reduced to their basest form and have begun fighting and hunting each other for sport. The Professor leads a group of SHIELD agents and X-Factor to Muir in an attempt to stop the Shadow King's machinations. Some X-Men, led by Forge, have fought against the negative influence, but the only way to truly defeat the Shadow King is on the Astral Plane. Professor X goes in but can't defeat the Shadow King himself. Fortunately, his X-Men show up to help protect him. Things look hopeless, however, until Forge finally finds a way to destroy the nexus that anchors the Shadow King to both the physical and astral existences. The Shadow King shrinks into the void and leaves the X-Men to pick up the pieces.
The Muir Island X-Men
Polaris gets her magnetic powers back and she, Madrox, and Guido are asked by Val Cooper to join a new, government sponsored mutant group. Legion, who had been possessed by the Shadow King, lies in a coma. Siryn leaves everyone and eventually joins X-Force. Since most of Muir Island was destroyed, Moira goes to live at Xavier's mansion for a time. Forge didn't pick a new group to be affiliated with but also went to the X-Mansion and worked on the technical affairs. Banshee also joined the rebuilding effort without truly joining the X-Men.
The five members rejoined the X-Men. X-Factor then became the new Freedom Force, a government sponsored team that would be headed by Havok.
With so many X-Men, the team split into two teams. Cyclops lead the Blue Team, consisting of Wolverine, Gambit, Psylocke, Rogue, and Beast, while Storm lead the Gold Team, consisting of Angel, Jean Grey, Iceman, Colossus, and the soon-to-be-introduced Bishop.
Also, the Shadow King had crushed Xavier's legs, leaving him confined to a wheelchair once again.
X-Men by Claremont and Lee Omnibus #2
X-Men Visionaries: The Art of Adam and Andy Kubert