Complex sentence: In the "Marvel 1602" miniseries, Jean Grey fakes her gender and poses as "John Grey" as a member of the "witchbreed" led by Carlos Javier (the Charles Xavier of the 1602 universe).
Split into simpler sentences: In the "Marvel 1602" alternative universe miniseries, Jean Grey fakes her identity (and gender), posing as "John Grey", a member of the "Witchbreed". The group was led by Carlos Javier (the Charles Xavier of the 1602 universe).