During their record-selling run on X-Men in the 1990s, Jim Lee and Whilce Portacio introduced the idea that there was a traitor on the X-Men who would betray his or her teammates in the near future and become a (very bad) legend in the distant future that the mutant known as Bishop grew up in. The X-Traitor became a major mystery for fans that was eventually resolved by different writers as part of the crossover, Onslaught. Now, though, there is a new X-Traitor, only this "traitor" isn't really what they appear.
Source：https://www.cbr.com/marvel-x-men-x-traitor/
