When the MCU’s Devil is introduced, every heretofore disproved fan theory about Mephisto could be proven correct. Many Phase 4 fan theories have focused on Marvel villain Mephisto, who is one of Marvel’s most well-known villains.
When he was first introduced, he was believed to be Evan Peters’ fake Pietro, the on-screen husband of Agatha Harkness’s bunny Seor Scratchy, or the real villain behind the events of WandaVision. Before Spider-Man: No Way Home was released, early speculations proposed the idea that Peter Parker went to Mephisto for a solution to his secret identity issues, with the Devil’s imagery in the Loki trailers suggesting that Mephisto was behind the TVA.
Mephisto is a popular character in MCU fan theories for a reason. Many Marvel events have been touched by Mephisto over the years, including the creation of the Infinity Stones; the formation of the Stone Age Avengers, World War II, and the death of Doctor Doom’s mother, Norman Osborn becoming Green Goblin, and Doctor Octopus regaining his original body. Mephistopheles has even appeared in the comics as a villain in his own right. Scarlet Witch used a portion of Mephisto’s soul to give birth to Tommy and Billy Maximoff, while the demon allowed Peter Parker to remove any knowledge of his secret identity from the public.
Debunked theories about Mephisto have been mocked for their persistence by MCU fans. There are some theories that can be confirmed in retrospect thanks to Mephisto’s appearance in the MCU. It is possible that Mephisto’s MCU debut will reveal that he was involved in the births (and disappearances) of Scarlet Witch’s twins, Kang’s conquest of the multiverse, and Doctor Strange’s misfired spells, showing that he was devious enough to avoid detection. To be fair, Mephisto has a history of secretly influencing history in order to amuse himself, so this makes sense.
There is little doubt that Mephisto will appear at some point in the near future. There may be a prequel to a Ghost Rider film or series in which Johnny Blaze or Robbie Reyes receives the Spirit of Vengeance from Mephisto. This dark corner of the MCU has been pioneered by films and shows like Moon Knight, Doctor Strange in the Multiverse of Madness, Blade and the upcoming Disney+ Halloween special It would be more exciting to learn that some of the most popular MCU fan theories were correct, at least in part, when Mephisto makes his MCU debut.
In order to make the retcon work, the storylines in WandaVision, Loki, and NWH would have to be stretched, which could weaken the impact of already established events. This is the biggest risk. However, Mephisto’s MCU debut could be one to remember if future MCU films make good use of previous fan theories. There is no way to prove or disprove Mephisto’s existence in the MCU, just like in the comics.