A third teaser trailer of Avengers: Doomsday has leaked online, offering the clearest indication yet of the X-Men’s entry into the Marvel Cinematic Universe (MCU).

The latest leak follows earlier unauthorised releases of individual promos hinting at the returns of Chris Evans and Chris Hemsworth. The newly surfaced teaser shifts focus to the X-Men, whose characters are set to appear in the MCU after remaining outside it for decades.

The teaser features Magneto and Professor X making their MCU debut, with a widely discussed shot of Cyclops drawing attention online. The X-Men films were previously produced by 20th Century Fox after the studio acquired the rights to the characters from Marvel in the 1990s.

Several social media accounts posted the teaser late Sunday night. The footage appears to have been recorded on a phone inside a movie theatre. It shows the Xavier School for Mutants in flames, accompanied by a voice-over from Patrick Stewart reflecting on who survives the battle.

Stewart’s Professor Charles Xavier is seen interacting with Ian McKellen’s Magneto, while brief shots show Sentinels causing destruction. The teaser also includes a sequence of Cyclops unleashing what appears to be his most powerful optic blast yet on film.

While the original clip was of low quality, AI-enhanced versions featuring higher-resolution footage and stills began circulating widely on social media by Monday morning.

Avengers: Doomsday will feature Robert Downey Jr as the villain Doctor Doom in a multiverse-focused film. Other cast members include Anthony Mackie, Danny Ramirez, Sebastian Stan, Paul Rudd, Tom Hiddleston, Letitia Wright, Winston Duke, Simu Liu, Pedro Pascal, Vanessa Kirby, Joseph Quinn, Ebon Moss-Bachrach, Florence Pugh, David Harbour, Wyatt Russell, Hannah John-Kamen and Lewis Pullman.

The film is scheduled for release in December 2026.