Mephisto is one of the biggest Marvel villains the Marvel Cinematic Universe has yet to introduce, though they have a variety of avenues to do so. Essentially the equivalent to The Devil in the Marvel universe, Mephisto is a powerful lord of evil ruling the underworld with an iron fist, hell-bent on finding pure souls to torture and corrupt for his own sick pleasure. A powerful, supernatural entity older than the Earth itself, the MCU timeline has thus far had a difficult time integrating him into the films’ lore.

https://screenrant.com/mcu-mephisto-multiverse-saga-introduce-after-theory/