Marvel Already Revealed How They’ll Include the X-Men in Avengers: Doomsday

Marvel Studios stunned fans worldwide with their elaborate cast reveal for Avengers: Doomsday, confirming the Fox-era X-Men are joining the Marvel Cinematic Universe. Through a painstakingly slow five-hour livestream, Marvel Studios unveiled a star-studded lineup featuring not just current MCU heroes but an unprecedented assemblage of mutants, including Patrick Stewart (Professor X), Ian McKellen (Magneto), Alan Cumming (Nightcrawler), Rebecca Romijn (Mystique), James Marsden (Cyclops), and Kelsey Grammer (Beast). While this crossover might seem sudden, the MCU’s recent storytelling has actually laid the groundwork for this collision of alternate universes. The breadcrumbs were there all along, with the most revealing clue hiding in plain sight during the post-credits scene of 2023’s The Marvels.

Before we enter speculation territory, we must review the MCU’s concept of Incursions, first introduced in Doctor Strange in the Multiverse of Madness. Dr. Reed Richards (John Krasinski) of Earth-838 explained them as universe-destroying collisions that occur when the boundary between realities is breached. The film shows the devastating consequences of such an event in the destroyed reality where Sacred Timeline’s Doctor Strange meets an alternate version of himself in a universe that’s slowly fading away after an Incursion takes place. In short, the aftermath of an Incursion leads to the erasure of entire timelines, a fate worse than death. Yet, while Incursions seem to be the biggest threat in the Multiverse, not many people are aware they even exist.

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In The Marvels’ post-credits scene, Monica Rambeau (Teyonah Parris) awakens in what appears to be the X-Mansion from Fox’s X-Men films, greeted by a variant of her mother Maria (Lashana Lynch) and none other than Kelsey Grammer’s Beast. This strategic placement of Grammer’s Beast serves multiple narrative purposes. First, it legitimizes the Fox X-Men universe as an actual alternate reality within Marvel’s larger cinematic multiverse. Second, Monica’s interdimensional journey effectively created awareness of the multiverse in that reality, likely prompting the X-Men to begin investigating these phenomena. Just like that, the MCU has the perfect narrative bridge that allows the X-Men to plausibly appear in Avengers: Doomsday without feeling forced or contrived.

The Incursion Event: How the Fox X-Men Universe Will Collide With the MCU

Based on Marvel’s established multiverse mechanics and the recent casting announcements, Avengers: Doomsday is likely to feature a full-scale Incursion between the MCU’s Sacred Timeline and the Fox X-Men universe. This catastrophic collision of realities provides the opportunity to bring these iconic mutants into the MCU while maintaining continuity with both franchises.

The script practically writes itself: following Monica Rambeau’s arrival in their universe, the X-Men, led by Professor X and Beast, begin investigating multiversal phenomena. Meanwhile, in the MCU, Doctor Doom (Robert Downey Jr.) emerges as a threat, potentially manipulating or accelerating Incursions to serve his own ends. As the boundaries between realities deteriorate, the X-Men and Avengers find themselves thrust together, initially as adversaries who misunderstand each other’s intentions before uniting against the common threat Doom represents.

The fact that Marvel Studios is casting younger actors for a separate MCU X-Men film also suggests a bittersweet fate for these Fox-era mutants. Their inclusion in Doomsday likely serves as both homage and farewell, with many potentially sacrificing themselves during the Incursion crisis. This would create emotional resonance for longtime fans while clearing the path for the MCU’s new generation of mutants to emerge in the aftermath of Secret Wars, perhaps as variants or amalgams of their Fox counterparts.

Marvel Studios president Kevin Feige has been strategically preparing audiences for this transition, stating that Deadpool & Wolverine marked “the beginning of that, and every [movie] post-Deadpool & Wolverine will be the Mutant era coming into the MCU.” He further explained that “the whole story of Secret Wars really leads us into a new age of mutants and of the X-Men,” suggesting the current Multiverse Saga was designed to facilitate this integration. This approach mirrors how Marvel Comics handled similar universe-merging events, particularly 2015’s Secret Wars, where various realities collided before being reconstituted into a new, combined universe. By adapting this concept, Marvel Studios can elegantly solve the narrative challenge of integrating the X-Men while still honoring what came before.

Avengers: Doomsday has officially begun production and is scheduled for release on May 1, 2026, followed by Avengers: Secret Wars on May 7, 2027.

How do you think the OG X-Men live-action cast will be added to Avengers: Doomsday? Share your theories in the comments!