The OG X-Men Are Finally in the MCU: Every Word from Downey, Grammer, and Romijn on Their Avengers: Doomsday Return

The first major crossover between the original X-Men trilogy and the Marvel Cinematic Universe is finally here. At San Diego Comic-Con 2026, the cast of Avengers: Doomsday — including Robert Downey Jr. as Doctor Doom, Kelsey Grammer as Beast, and Rebecca Romijn as Mystique — gathered for a historic Hall H panel and a post-show interview with Entertainment Weekly. Here is everything the actors said, plus every confirmed detail from official sources about the December 18 film and the September 25 Endgame Encore re-release that sets it up.

Robert Downey Jr.: “We nerded out” with the original X-Men

Speaking to Entertainment Weekly immediately after the Comic-Con panel, Robert Downey Jr. described his reaction to seeing Grammer and Romijn back in costume. “I have to say, we nerded out,” Downey said. “It was a big moment, you and Kelsey being back.” He added that the original X-Men cast “lit up this constellation of excitement” on the Doomsday set . Downey, who now plays Victor von Doom after a decade as Tony Stark, told the actors that their return was one of the most thrilling parts of the production .

Kelsey Grammer and Rebecca Romijn: Enthusiastic about the MCU debut

Kelsey Grammer (Beast) and Rebecca Romijn (Mystique) were both present at Comic-Con alongside other original trilogy cast members — James Marsden (Cyclops), Famke Janssen (Jean Grey), Patrick Stewart (Professor X), and Ian McKellen (Magneto). In the EW interview and on the Hall H stage, they expressed excitement about returning to these characters more than 25 years after the original X-Men (2000) launched the modern superhero movie era . The reunion was the first time the original X-Men had assembled publicly for Doomsday and the first time they will appear in a Marvel Studios film after Disney’s acquisition of Fox .

The original X-Men Comic-Con reunion: Historic first for the MCU

At San Diego Comic-Con 2026, Marvel Studios split the massive Hall H cast across all four corners of the stage, with the X-Men (Marsden, Romijn, Grammer) and the Thunderbolts/New Avengers in the back rows . The panel and the subsequent EW group interview marked a historic moment — the first time characters from the original X-Men trilogy appear in a Marvel Studios film after the Disney-Fox merger . “More than 25 years after X-Men helped launch the modern superhero movie era,” reported People, “the franchise’s original stars reunited ahead of their highly anticipated return” .

Doctor Doom’s backstory: New details from Comic-Con and leaks

Significant new details about Doctor Doom’s origin emerged from Comic-Con footage and earlier reports:

• Collider reported that the Comic-Con footage centers on Doom, positioning him as the MCU’s next “big nasty,” with the film teasing a conflict that spans the Avengers, Fantastic Four, X-Men, and Thunderbolts .
• Leaked story details reported by The Direct indicate a major change to Doom’s classic backstory: Victor von Doom’s wife and son are killed in an accident, Doom himself is injured, and he traces the cause of the accident to Steve Rogers (Chris Evans) going back in time at the end of Endgame. Doom then targets Steve to avenge his family .
• Bleeding Cool noted that Marvel is reprinting the Infamous Iron Man comic series — a run featuring a version of Doom taking up the Iron Man mantle — as promotional setup for Downey’s dual-casting (reported by ScreenRant) .
• Both leaked and reported sources suggest Doom’s origin is being retooled to directly tie into the timeline-altering events of Endgame, making the Endgame Encore re-release a narrative bridge .

Note on certainty: The Doom backstory details about his wife and son come from leaked sources and trade rumor reporting (The Direct, Bleeding Cool) rather than official Marvel announcements. While consistent across multiple reports, these plot points should be treated as unconfirmed until the film’s release or an official synopsis.

Friction between superhero teams: Avengers vs. X-Men

Cast members have openly described tension and direct conflict between the assembled teams:

• David Harbour (Red Guardian), speaking to EW at Comic-Con, confirmed there is “trouble” between the New Avengers (from Thunderbolts) and the original Avengers. “I mean, clearly, we are right, but not everyone agrees with us,” Harbour said, hinting at ideological clashes on set .
• The Direct reports that a CinemaCon trailer showed three specific Avengers — including Thor, Sam Wilson/Captain America, and Shang-Chi — actively fighting the X-Men, confirming that the two teams will come to blows before uniting against a common threat . The trailer reportedly included a sequence of Shang-Chi battling Gambit (Channing Tatum), Yelena Belova fighting Mystique, and Storm facing Doctor Doom .

Avengers: Endgame Encore — September 25, 2026 re-release

Marvel Studios has officially titled the theatrical re-release Avengers: Endgame Encore, arriving on September 25, 2026 .

• The re-release will feature approximately 7 minutes of new footage, including a brand-new ending sequence directly connecting to Avengers: Doomsday .
• The Russo Brothers confirmed at CinemaCon that the new footage includes a custom introduction and a post-credits tag linked to Doomsday .
• The re-release is being presented in a format Disney is calling “Infinity Vision” — a premium large-screen standard featuring advanced laser projection and immersive audio .
• Official confirmation comes from Marvel.com as well as multiple major outlets .

Avengers: Doomsday — Release date

Avengers: Doomsday is scheduled for theatrical release on December 18, 2026 . It is directed by Anthony and Joe Russo, written by Michael Waldron and Stephen McFeely, and features an ensemble cast including Robert Downey Jr. (Doctor Doom), Pedro Pascal (Reed Richards), Chris Hemsworth (Thor), Anthony Mackie (Captain America), Sebastian Stan (Winter Soldier), and the original X-Men cast . The film will be followed by Avengers: Secret Wars on December 17, 2027 .