A Class Two Demon or Hell-lord, Mephisto rules a fiery nether realm that he refers to as Hell or Hades, though it is neither the Biblical Hell nor Pluto's Olympian Hades. In the distant past, Mephisto became engaged to the lover of one of his former soldiers, then manipulated that demon into slaying his lover; the demon became Darklove, cursed to always perform the opposite of love. Mephisto's realm includes the Arena of Tainted Souls where tainted souls are locked in perpetual combat. Mephisto delights in impersonating the Biblical Satan. Empowered by the souls of the damned, he often strikes deceptive bargains with the living. Mephisto takes particular interest in the souls of extremely powerful or exceptionally pure beings.
Long ago, Mephisto entered a rivalry with the demonic Zarathos and struck a bargain with the young prince who eventually became Centurious, the Soulless Man. Together, they brought down Zarathos, and Mephisto began a long process of tormenting Zarathos by bonding him with mortals. Knowing Zarathos was connected to the ancient Spirits of Vengeance, Mephisto became obsessed with controlling the Medallion of Power, a mystic talisman housing the essences of the Spirits. This led Mephisto to the Kale family, who bore shards of the Medallion within their souls. In the 18th century, Mephisto bargained with the overzealous madman Pastor Kale, hoping to gain the soul of the Pastor's son, Noble. This eventually led to the creation of the Dan Ketch Ghost Rider.
Throughout history, Mephisto has exploited the spiritually adrift. In Europe, he granted a group of mystics (the future Mys-Tech) immortality in exchange for a yearly quota of souls. He also corrupted sea captain Joost van Straaten, who became the infamous Flying Dutchman. Benedicto de vica Severtes once dealt with Mephisto to gain the love of Beatriz, and prevented Mephisto from claiming his soul by encasing himself in mystical armor, becoming the Brass Bishop, but after centuries of evading Mephisto, his soul was finally claimed when the hero Puck cracked his armor. Prior to World War II, Mephisto duped a group of German scientists into believing in the Vril, a perfect race that had allegedly ruled Earth before the age of man and had relocated to an extradimensional realm. These scientists developed a portal to the Vril, though their project was shut down at war's end. Surviving project member Thule secretly continued this work in the Rocky Mountain base Schamballah, assisted by a growing band of neo-Nazis.
In recent years, Mephisto has been increasingly fascinated by Earth's super-beings, and has particularly coveted the Silver Surfer's pure spirit. Mephisto tried to tempt the newly Earth-bound Surfer with power and physical pleasure, even offering up the Surfer's beloved Shalla-Bal, but the Surfer resisted. Undeterred by this failure, Mephisto has repeatedly threatened the Surfer, Thor, Daredevil and many other heroes. He sent the ghostly Flying Dutchman back to Earth to attack the Surfer, but the Dutchman renounced Mephisto. When Noble Kale's descendant Naomi Kale sought to undo the Kale family curse, she offered Mephisto her soul in exchange for his promise that the curse would not plague her firstborn son. She hoped this might break the curse forever, but Mephisto taunted her with assurances that even if her son John could not become "the" Ghost Rider, he could certainly become "a" Ghost Rider. Years later, when John Blaze turned to the occult in a futile attempt to save the life of his adopted father, Mephisto appeared to him in the guise of Satan. Claiming Blaze's soul, Mephisto bonded the human with the captured Zarathos as a new Ghost Rider.
Mephisto's power over the soul of Doctor Doom's mother led him into several conflicts with the Latverian monarch, battling each other annually on Halloween. Allied with fellow Hell-lords Satannish, Thog and Satan, Mephisto manipulated the demonic Six-Fingered Hand into helping him create Hell on Earth, but the non-team known as the Defenders thwarted their efforts. After Thor freed mortal souls from his Hell, Mephisto attempted to slay Thor's then-human form, Dr. Donald Blake, employing the deceased Darkoth against him, but Darkoth had no love for Mephisto, and had Thor destroy him. Mephisto has menaced the Fantastic Four on several occasions, including one involving the demonologist Elspeth Cromwell who mistakenly invoked him, allowing him to capture Reed and Susan Richards and their son Franklin.
Mephisto had gained tremendous power by the evil of the extraterrestrial Dire Wraiths who were invading Earth at the time, but lost this power when the spaceknight Rom banished the Dire Wraiths, and Mephisto was temporarily destroyed by Franklin. Mephisto later re-formed, missing five fragments of his essence. He tricked the villain Master Pandemonium into gathering these fragments, which Pandemonium believed to be pieces of his own lost soul, and Mephisto was eventually restored to full strength. This resulted in the destruction of the Scarlet Witch's twin sons, whom she had mystically created by unwittingly trapping and transforming two of Mephisto's power fragments. A rivalry developed with the Asgardian death goddess Hela, who stole several souls that Mephisto considered his. Scheming to force Thor to oppose Hela, Mephisto temporarily stole the souls of several heroes, including the Invisible Woman, Jean Grey and Rogue, before finally claiming the one he'd sought all along: the Thunder God's. After a series of confrontations with the Fantastic Four, X-Factor, the X-Men and the Avengers, Mephisto freed Thor's soul, but his plot succeeded. Having seen the horrors in store should the realm of death ever expand to engulf the living, Thor vowed to stand against Hela's advances forevermore.
With at least one rival neutralized, Mephisto sought other means of increasing his power. When the godlike Beyonder came to Earth seeking understanding of humanity, Mephisto attempted to sway him by offering his own warped view of mortals, sending Zarathos to test Spider-Man's morality. When Mephisto tricked the herald of Galactus, Nova, into becoming more bloodthirsty in her duties, the Silver Surfer and Galactus both opposed the demon. Galactus and Mephisto engaged in a cosmos-shaking battle that left the Hell-Lord badly beaten. More failed attempts to gain the Medallion of Power followed, as well a brief period in which Mephisto served as an advisor to Thanos, who then held the Infinity Gauntlet. Mephisto lied to Thanos about his origins, linking them to the Infinity Gems, in the hopes of gaining the villain's trust. Thule's Eye of Force eventually achieved its goal, opening a portal to the "Vril," only for them to be revealed as Mephisto's demon elite. Mephisto devoured Thule as payment, but his intended invasion of Earth was thwarted by Cloak and Dagger, Spider-Man, and Ghost Rider (Dan Ketch).
Mephisto once stole the soul of Odin while he was in his Odinsleep, and Loki possessed Odin's form, but Odin ultimately reclaimed his body and Mephisto took Loki's soul, his true goal all along. After months in Hell, Loki's spirit eventually escaped. Mephisto empowered suicidal Chris Daniels into the superhuman Suicide, cursing him so that only the Ghost Rider (Ketch) could slay him, hoping that Suicide could force Ketch into committing murder. The demon later attempted to seduce the Silver Surfer by mimicking the form of Nova (Frankie Raye), and Mephisto ruthlessly murdered Avengers member Mockingbird after the Avengers interfered in Mephisto's clash with rival demon Satannish over the souls of the Lethal Legion. Mephisto later helped the hero Hellcat destroy Satannish.
Appreciating the strength mortals drew from family and friendships, Mephisto mystically created two children for himself: a son named Blackheart, whom he deemed worthy of being his heir, and a daughter named Mephista. Mephista would wage war upon Doctor Strange, but Blackheart targeted first Daredevil and later the Dan Ketch Ghost Rider. Mephisto sought to teach his son the ways of ruling Hell, but Blackheart's ambition began to rival his own and they turned on each other. Blackheart temporarily overthrew his father and claimed the throne of Hell, but he eventually lost it to Noble Kale, who was in turn cast out by a renewed Mephisto. After reclaiming his throne, Mephisto once more began focusing on acquiring more souls for his realm. His efforts to claim the Black Panther (T'Challa) were thwarted when he was nearly overwhelmed by all of the Black Panther spirits throughout history. The Haazareth demons who granted Doctor Doom magical power against the Fantastic Four are within Mephisto's domain, and tormented Doom for their master when he offended their patronage.
Mephisto took advantage of Spider-Man's desperation to save his Aunt May's life, after she was shot by one of the Kingpin's snipers. In exchange for her restored health, Spider-Man had to sacrifice his marriage to Mary Jane Watson, as the demon would revel in the elimination of a sacred and holy bond of love, and Spider-Man's innate suffering (despite losing memory of the event). The couple agreed, and Mephisto altered time such that the Parkers never exchanged wedding vows. As part of the deal, Mephisto removed the knowledge of Spider-Man's secret identity from the world. Through this event, Harry Osborn was somehow restored to life.
* NOTE - In the alternate future timeline of the Guardians of the Galaxy, Mephisto has a loyal daughter named Malevolence.