Mephisto

Mephisto is not the Devil, though he is happy for you to think so — he is an ancient demonic entity who rules a pocket dimension shaped like every hell humanity has imagined, and who feeds on despair and stolen souls. His power is enormous, but he almost never uses it directly. Mephisto's method is the deal: a genuinely tempting offer, honoured to the letter, that ruins the person who accepts it in a way they only understand later.

That method has cost Marvel's heroes more than any invasion. He made Johnny Blaze the Ghost Rider, tormented the Silver Surfer for the sin of being incorruptible, and in One More Day (2007) he took Peter Parker's marriage to Mary Jane in exchange for Aunt May's life — the most controversial editorial decision in modern Marvel history and, whatever you think of it, the most consequential thing any villain has ever pulled off. He is the villain Marvel reaches for when a hero needs to lose something that cannot be won back in a fight.

Mephisto debuted in Silver Surfer #3 (1968), targeting the Surfer specifically because a being that pure offended him. He became Marvel's default infernal power broker, tied to Ghost Rider's origin, Doctor Doom's mother, Blackheart, and the Hell-Lord politics that run through Doctor Strange and Midnight Sons stories.

One More Day (Amazing Spider-Man #544-545 and related, 2007) is his defining modern act: he erases the Parker marriage in exchange for Aunt May's survival, resetting Spider-Man's status quo and igniting a fan argument that has run for nearly two decades. Marvel has since used him in Avengers, Fantastic Four and Scarlet Witch stories as the ultimate cost-imposer.