The Marvels ended with one of the group’s three members lost in a parallel universe. The jump point Dar-Benn created over Earth grew even more unstable after she tried to use both Quantum Bands at the same time. The resulting explosion caused a cosmic power overload that not only killed Dar-Benn, it caused a rip in space itself. It opened a door to a different reality which then began bleeding into the MCU’s Earth-616. Monica Rambeau alone stopped a potential universe-destroying Incursion by absorbing the combined energy of Kamala Khan and Carol Danvers. That gave her the power she needed to close the tear using her electromagnetic spectrum manipulation abilities. But Monica knew her heroism would come with a cost. She was only able to close the hole from the other side. When it safely shut she was stuck in a completely unknown branch of the multiverse.
At least it was completely unknown until the film’s only post-movie sequence. A mid-credits scene revealed Monica ended up in a reality with major implications for the MCU’s future. Monica woke up in a hospital bed after someone rescued her from space. Sitting by her side was a Variant of her mother, Maria Rambeau. That Maria doesn’t have a daughter, though. She’s not a mom; she’s a superhero known as Binary, a persona that belonged to Carol Danvers in Marvel Comics.
More importantly, Binary’s associate and Monica’s doctor, turned out to be someone who reports to “Charles.” The scene featured Kelsey Grammer’s Dr. Hank McCoy, a.k.a. Beast, from Fox’s X-Men franchise. His surprise appearance is the most consequential moment yet for mutants in the MCU. It means the X-Men have finally joined the series for good.
Ms. Marvel‘s season one finale revealed Kamala Khan to be the MCU’s first confirmed mutant. Since then others like her have officially joined the franchise. A Variant of Patrick Stewart’s Professor Charles Xavier appeared in Doctor Strange in the Multiverse of Madness. (That film also featured the first Maria Rambeau Variant. She was that dimension’s Captain Marvel.) Namor was the main antagonist of Black Panther: Wakanda Forever. And Mr. Immortal made a brief cameo on She-Hulk. There might also be more hiding in plain sight unaware of their genetic mutations.
However, none of those characters have direct ties to Fox’s X-Men films, which are poised to play a major role in the MCU soon. The version of Charles Xavier from Multiverse of Madness wasn’t just a Variant, it’s not clear any other X-Men or mutants exist in his reality. It’s also not yet known what connections, if any, the version of Hugh Jackman’s Wolverine who appears in Deadpool 3 will have with his original Fox universe character. Despite mutants’ presence, the X-Men didn’t exist in the MCU until The Marvels post-credits scene. Now they do, because in that universe Beast reports to “Charles.”
We don’t yet know if the reality Monica ended up in is the exact same universe as the Fox X-Men movies, but The Marvels strongly hinted at it. In addition to Grammer’s Beast, the scene featured music (listed in the credits) from both X2 and Days of Future Past. That’s a big difference from the musical cue we got for Kamala Khan on Ms. Marvel. A motif from the X-Men animated series accompanied her genetic mutation reveal. If Marvel wanted us to treat Beast, Binary, and the unseen Charles Xavier as just other mutant Variants, why use Fox-era music?
The presence of Maria Rambeau’s Binary and the difference in Beast’s physical appearance means we can’t definitively say Monica ended up in the Fox X-Men universe. But no matter which exact reality she’s now trapped in isn’t as important as what that parallel world means for the MCU. The X-Men are now, finally, an official part of the franchise. They’ve also met a hero of Earth-616 and now know, impossible as it seemed to Hank McCoy before, they are not alone in the universe.
It’s only a matter of time before they meet more of a different Earth’s Avengers.