In the earlier version of the X-Men movie canon, Jean was recruited by both Magneto and Professor X, who placed a mental block in her mind because her powers were too dangerous for her to use. The third movie, X-Men: The Last Stand, showed Jean losing control and causing
widespread destruction thanks to her split personality, known as the Phoenix.
As Days of Future Past effectively erased the events of The Last Stand, it seems that Apocalypse director Bryan Singer is having another shot at the Phoenix storyline from the comics – or at least, he plans to tease it. Fan speculation argues that Jean’s visions of the end of the world suggest a darker direction for the character, and trailers which center around Jean suggest that the character (and her formidable psychic Phoenix power) may be crucial in defeating Apocalypse. Either way, there’s likely to be more than a few references to the dark power within Jean in the upcoming movie.