In Marvel Comics, Onslaught was a sentient psionic entity that was created by combining the consciousnesses of Professor X and Magneto, becoming an amalgamation of the dark corners of both of the powerful mutants’ minds. Onslaught sought to spread fear across the world in a twisted attempt to unify humans and mutants, waging several psychological attacks on the X-Men before eventually going head-to-head with the combined forces of the X-Men, the Fantastic Four, the Avengers, and Doctor Doom. Onslaught’s eventual defeat led to the apparent deaths of several of the Avengers, the Fantastic Four, and Doctor Doom, though the heroes were kept safe on Counter-Earth.

Marvel Comics’ Onslaught crossover storyline has been speculated to be coming to the MCU thanks to Patrick Stewart and Sir Ian McKellen’s rumored involvement in Deadpool 3, which creates the opportunity to feature Doom initially as a hero. However, it seems unlikely that someone as major as Doctor Doom would be introduced during Deadpool 3, especially considering the multiversal film promises to be jam-packed with action and returning characters from Fox’s X-Men franchise. Instead, Doom’s introduction might be better suited for later in Phase 6, potentially even in Avengers: The Kang Dynasty, which could see him defeat Kang the Conqueror and take his place as the MCU’s next big bad.