The Lucas Bishop, a time traveling mutant from a dystopian future. Bishop's brutal methods immediately brought him into conflict with the X-Men, but it wasn't long before he'd actually ed up, giving the Gold Team the kind of dark edge Wolverine gave to the Blue Team.
Bishop's arrival in the present day also saw the launch of the infamous "traitor" plot. Uncanny X-Men #287 revealed the X-Men were just legends in Bishop's time, but that he had personally learned there was truth to the myths when he stumbled upon the ruins of Xavier's Mansion. There, he found a recorded message featuring Jean Grey, which revealed the X-Men were destined to be betrayed by one of their own. This was initially just a historical curiosity to Bishop - until he traveled back to the present, and became obsessed with averting his dystopian timeline.
There was just one problem; the writers had absolutely no idea what to do with this. Back in 2010, Jim Lee and Whilce Portacio told Image Comics, including Jim Lee himself, and whatever plans were in place for the X-Men swiftly changed.
Years ed, and Marvel began working on the "Onslaught" epic, eventually deciding to have Charles Xavier himself go rogue and become a devastatingly powerful enemy for the X-Men. This was logically seen as the resolution of the "traitor" arc, and indeed Marvel even showed a version of Jean Grey recording this message - with the missing words filled out. It was fairly cumbersome because Uncanny X-Men #287 clearly suggested only a few key parts of dialogue were missing due to the degradation of the message, whereas the final version filled in entire chunks. Ironically, the message served only to unintentionally highlight the fact that the original plans had been changed.
Making matters worse, "Onslaught" was hardly a cohesive story in its own right; the various creative teams seemed to be pulling in different directions, with differing s of just how this psychic being had been created in the first place. Had a fragment of Magneto's psyche implanted itself within Xavier's mind when they were in battle, or was Onslaught simply the manifestation of all Xavier's repressed inner darkness, even including his old infatuation with Jean Grey when she first became his student? The writers don't seem to have had a common view, and as a result the story feels mismatched and confusing. That meant one of the most interesting ideas in X-Men history - that the X-Men were destined to be betrayed by one of their own - had been grafted into a poorly-constructed, ill-thought-through epic that just didn't work. It really was a waste of tremendous setup.
