Modern X-Men stories have highlighted what makes Mister Sinister the mutant team's most persistent and dangerous foe. The evil genius has pushed the limits of his own capabilities, and forced the entire Marvel Universe to a breaking point. After Sins of Sinister, it seemed he was finally paying for his crimes once and for all, but a surprising turn of events in Fall of X teases that he may return sooner rather than later.
Source：https://www.cbr.com/x-men-fall-of-x-mister-sinister-tease-marvel/
