The mutants are here and ready for battle. In the third official teaser for Marvel Studios' upcoming epic "Avengers: Doomsday," the original X-Men from the Fox film franchise officially join the Marvel Cinematic Universe. The somber clip, currently playing in theatres, spotlights iconic actors Ian McKellen as Magneto, Patrick Stewart as Charles Xavier, and James Marsden returning as Cyclops.

The teaser opens on the wreckage of Xavier's School for Gifted Youngsters, setting a tone of loss and impending doom. It features a poignant moment between Professor X and Magneto before cutting to an emotionally charged Cyclops unleashing a devastating optic blast, with what appear to be massive Sentinel legs looming in the background. Magneto's voice-over asks a haunting question: "The question isn't 'are you prepared to die?' The question is 'who would you be when you close your eyes?'"

This marks the Marvel Cinematic Universe debut for McKellen and Marsden. Stewart previously appeared in a variant role in "Doctor Strange in the Multiverse of Madness," but this reunites the core trio from the original "X-Men" (2000) film. They will join a vast ensemble that includes returning Avengers like Chris Evans' Captain America, Robert Downey Jr. as the villain Doctor Doom, and the heroes of the upcoming "Fantastic Four."

Directed by Anthony and Joe Russo, "Avengers: Doomsday" assembles heroes and mutants for a battle that will define the future of the MCU when it hits theatres on December 18, 2026.