Mastermind is a supervillain appearing in American comic books published by Marvel Comics, commonly as an adversary of the X-Men. The original Mastermind was a mutant with the psionic ability to generate complex telepathic illusions at will that cause his victims to see whatever he wishes them to see. He was a founding member of the first Brotherhood of Evil Mutants and later a probationary member of the Lords Cardinal of the Hellfire Club, where he played an important role in "The Dark Phoenix Saga".

The X-Men are a superhero team appearing in American comic books published by Marvel Comics. Created by writer/editer Stan Lee and artist/co-plotter Jack Kirby, the team first appeared in The X-Men #1. Although initially cancelled in 1970 due to low sales, following its 1975 revival and subsequent direction under writer Chris Claremont, it became one of Marvel's most recognizable and successful franchises. They have appeared in numerous books, television shows, 20th Century Studios's X-Men films, and video games. The X-Men title may refer to the superhero team itself, the eponymous comic series, or the broader franchise including various solo titles and team books such as the New Mutants, Excalibur, and X-Force.