Complex sentence: Only a select group of X-Men (Beast, Gambit, Nightcrawler, Psylocke, Wolverine, Cyclops) have avoided capture and, temporarily united with their oldest foe, Magneto, they must defeat the Phalanx before all of humanity is assimilated.
Split into simpler sentences: Only a select group of X-Men (Beast, Gambit, Nightcrawler, Psylocke, Wolverine, Cyclops) have avoided capture and, temporarily united with their oldest foe, Magneto. They must defeat the Phalanx before all of humanity is assimilated.