An appearance by the demonic character has been a popular fan theory since with almost every Marvel Disney+ series since being plagued by suspicion that Mephisto would be revealed to be the big bad.

Source: https://www.cbr.com/star-wars-solo-actor-joining-major-franchise-mcu-ironheart/

As the MCU grows more cosmic in scale, Mephisto has the opportunity to make his mark on stories such as these, putting behind him the days of the wild theories that can now actually come true.

Source: https://screenrant.com/mephisto-mcu-special-wandavision-theories-true/

For years, Earth's Mightiest Heroes have pushed back against the threat of Mephisto and his multitude of schemes.

Source: https://www.cbr.com/avengers-forever-mephisto-multiverse-broken-marvel/

In Avengers Assemble Omega, hundreds of universes have been destroyed thanks to Mephisto and the Masters of Evil.

Source: https://screenrant.com/10-biggest-marvel-dc-deaths-2023-ms-marvel/

In their fight against Mephisto, Earth's Mightiest Heroes have banded together alongside Avenger Prime and countless versions of themselves from across time and space.

Source: https://www.cbr.com/avengers-mephisto-council-of-red-betrayal-marvel/

The text inside the ring reads the mysterious quote, "Mephisto avenge this house".

Source: https://gamerant.com/assassins-creed-hexe-mephisto-reference-explained-witchcraft-demon/