Jessica Chastain's role in X-Men: Dark Phoenix has drawn a lot of speculation since she was cast, with many suspecting her role to be that of Shi'ar Empress, Lilandra, or even the Skrull queen, Veranke.
However, online chatter is now suggesting she may actually be playing the female version of Mister Sinister, aptly titled Miss Sinister. The rumors, courtesy of The Tracking Board, hints that Fox will be introducing her first as the X-Men's famous villain as opposed to the male version which is more prominent in the comics and cartoons.
Sinister, following Jean Grey's death in the Dark Phoenix Saga, cloned her as Madelyne Pryor, who then had a child, Cable, with Cyclops. It's unknown if this actually ties into Josh Brolin's appearance in Deadpool 2, but the villain was hinted at in the post-credits of X-Men: Apocalypse.
With Sophie Turner's Jean set to become the powerful entity once again in this reboot, if Chastain does play Sinister, then she could well end up following the character's obsession with the mutant's relationship with Cyclops. In the books, Sinister often transferred his mind to various incarnations of his body, and in 2008's X-Men Legacy #214, he activated this female clone, Claudine Renko, who then became Miss Sinister, which the rumors suggest is the path Chastain might well be following.
Opening Feb. 14. 2019, X-Men: Dark Phoenix stars James McAvoy as Charles Xavier, Michael Fassbender as Magneto, Jennifer Lawrence as Mystique, Nicholas Hoult as Beast, Sophie Turner as Jean Grey, Alexandra Ship as Storm, Tye Sheridan as Cyclops and Kodi Smit-McPhee as Nightcrawler.