For months an ominous name has haunted the X-Men... Onslaught. Who or what is this mysterious creature and why is he so obsessed with the mutant heroes? Meanwhile, as the team wrestle with this phantasmagorical foe, Jean Grey inches ever closer to discovering the identity of the supposed X-traitor — a hero prophesied to destroy the group from within. Could these two unknown assailants be somehow connected? Witness the event that rocked the X-Men to their core and reshaped the Marvel Universe for years to come!