Concluding a 15-year journey across the earth in 1349 A.D., Mortigan Goth summoned Mephisto to save him from the plague. Mephisto challenged Goth to a game of chess, gambling Goth's soul for immortality, and Mortigan won. In his hometown, he found his sister dead and mother dying, and the priest Nicholas de Bellrais offering to sacrifice his entire congregation to Mephisto for immortality. Goth offered his own soul to save the congregation, and Mephisto accepted, taunting Goth when the congregation all died from the plague. When Goth vowed to fight Mephisto eternally, the demon told him that for every hurt Goth dealt him, his soul would receive a thousand times more. Mephisto cursed Immortalis to see suffering, knowing he might help but certain he dare not. Goth eventually learned that Mephisto would torture his soul regardless, so he opposed the demon in subtle fashion.

Goth eventually befriended the Falsworths, but in 1940 he rejected a relationship with their distant cousin Katherine Ainsley-Jones, unable to bear watching a lover age. In a misguided effort to achieve immortality, she let Baron Blood (John Falsworth) vampirize her, though when she drank Goth's blood she was cured. In 1965 Goth helped Dr. Strange banish a demon from Mephisto's Hell which had possessed a young girl, taunting the demon until it emerged to attack him; he then warned Strange that the principalities he invoked for power would eventually demand favors in return, and also warned of vampires in his future. In 1993, Goth's friend Professor John Hinde used an hallucinogen to punish student Bentley, who had been selling the drug to students.

In the modern era, Mephisto released Goth's soul, maddened by centuries of torture; it began slaying beings granted immortality by Mephisto, who felt that most of his immortals servants led tedious lives or opposed him. Goth was initially blamed by the widow of victim Johannes Faustus. Having befriended the ghost of Tony, a man he had failed to save, Goth joined Strange against Katherine and her group of child vampires, using Jacqueline (Spitfire) Crichton's Human Torch-altered blood to destroy Katherine. Goth later confronted his corrupt soul, which tore out his left eye and told him it would kill him a bit at a time. Goth was happy; he now knew he could die, rendering life precious.