Chthon is the archetypal Class One Demon, one of the Elder Gods created billions of years ago from the Demiurge, the sentient life force of Earth's biosphere, shaping the early development of Earth. Chthon, like his sister Gaea was born a god of the Earth, but he was not of the lifegiving soul that she was, instead Chthon was a being of the dark cold soul of the grave and the underworld. Chthon was Earth's first master of black magic, occupying his existence in the pursuit of knowledge of things which should not be. As with the majority of Elder Gods, Chthon began degeneration into a demonic state, and when the benevolent Gaea mated with the Demiurge and spawned Atum, Chthon saw the threat this new "God-Killer" posed. As Atum began killing those Elder Gods who had become corrupted, Chthon inscribed all of his dark knowledge into indestructable parchments which would come to be known as the Darkhold. Chthon then weaved a spell allowing him to escape to a new dimension, leaving the Darkhold behind to serve as a touchstone to his home.
Trapped in this new realm, Chthon sired the N'Garai demons - including the masters (Kierrok, Syxa, the Triad (Y'Bsgloth, Y'Garon, Y'Griarth), etc.) and the unnamed warrior caste. Though the N'Garai have been able to return to the Earth dimension, Chthon himself was unable to. Acting through an elemental host called the Other, he granted power to the Darkhold's users, including the Atlanteans who would become the first vampires and later of the current incarnation of zombies (with the alliance of the voodoo god Sagbata (Baron Samedi)). All mortals who used the Darkhold forfeited their souls and were corrupted by Chthon, though Chthon was not a collector of souls like Mephisto, he instead used the mortals as pawns towards his own desire to return to Earth. The Darkhold was used by many including Babylonian savants, Egyptian priests, and Hebrew scholars. Chthon has also empowered or created numerous beings over the eons, including the Darkhold Dwarf, Diabolique's mother, Dragonus, Hellhound, Midwife, Monstrosity, Troids, Whisperer, and the Wish-Demon.
In the 6th century, Morgan Le Fay bound the Darkhold parchments into a book and summoned Chthon to Earth, but soon realized he was too powerful to control. Even aided by Merlin, she could not drive him from Earth completely; they imprisoned a portion of Chthon within what would become Wundagore Mountain. Soon thereafter, Chthon's elemental host, the Other was summoned by Modred the Mystic, who resisted corruption until he had to sacrifice himself to save the woman he loved. Modred fell into a deep sleep and his master Gervaisse put him into an enchanted tomb knowing he would one day awake.
The Other was fought off by Merlin and St. Brendan, who scattered the Darkhold's pages across the planet. The Darkhold was re-bound by the Mad Monk Aelfric in 1149 A.D. and passed through many hands, including sorcerers Cagliostro and Taboo and vampire Lord Ruthven. Many partial or complete copies of the Darkhold exist, including the Shiatra Book of the Damned and the diary of Grigori Russoff.
The Vatican came to aquire at least one of these copies and sought to keep it away from the world, shut it away in their libraries under the protection of the Montesi family, priests of a supposedly uncorruptable bloodline who were allowed to have children to continue watching the Darkhold.
In the 1930s, the High Evolutionary built his Citadel of Science atop Wundagore, which allowed Chthon to witness many scientific achievments of interest to him. Over 20 years later, Gregor Russoff summoned Chthon in hopes of curing his lycanthopy, but this led to the demon attempting once more to return to Earth. The Knights of Wundagore and the wizard Magnus drove him back; in parting, Chthon struck down Russoff and bonded to a newborn, the future Scarlet Witch.
More recently, Vittorio Montesi, a priest, found he was sterile and used the Darkhold to father a child, Victoria who was more a child of Chthon himself than of the priest.
In the modern era, Modred was awakened and Chthon took control of this pawn to capture the Scarlet Witch. The demon tried to return to Earth by taking over her, but her fellow heroes in the Avengers were able to rescue her. More recently, after a weakening of the mystical walls of the dimensions, Chthon sent the magical Dwarf to tempt others into using Darkhold pages, and Victoria Montesi was part of the "Darkhold Redeemers" opposing this activity. Among the many victims of temptation were Blade, Harvestor (Johnny Farmer), Mercy (Norman Zachos), and Spider-X. Chthon eventually gestated within Victoria, but Doctor Strange prevented him from being reborn on Earth by drawing on the goodness within Victoria's soul.