Born untold thousands or even millions of years ago in the dimension of the vastly powerful energy-entities called the Faltine, Dormammu and his sibling Umar gathered matter to themselves to increase their power, a practice anathema to their fellow Faltine, becoming their dimension’s mightiest sorcerers. Ultimately slaying their parent Sinifer, they were driven from their home dimension, the Faltine deeming their garnering of such energy intolerable, and assumed human form to explore others. Their search for conquests brought them to the Dark Dimension, a mystic realm riddled with natural warps into pocket universes within the dimension itself. At the time of their arrival, the Dark Dimension’s multi-species society had been at peace for almost 28,000 years and was in the third millennium of the rule of King Olnar. Despite the warnings of his sorcerer advisors the Mhuruuks, Olnar offered Dormammu and Umar sanctuary and power in exchange for the knowledge of how to shatter dimensional barriers and absorb other dimensions into his domain. After decades of such conquest, Dormammu, whether by chance or design, led Olnar to the dimension of the Mindless Ones, indestructible rampaging dreadnoughts who slew Olnar and many other Dark Dimension denizens. Dormammu and Umar repelled and confined the creatures, using the confusion to murder most of the Mhuruuks, and were hailed as heroes by the Dark Dimension, of which they were proclaimed regents; Olnar’s infant son Orini, deemed no threat, was allowed to live under their care. As ruler, Dormammu partially reverted to flaming Faltine form and merged his essence with that of the Dark Dimension’s innate mystic energies, increasing his power enormously.
Little is known of Dormammu’s early rule, and since denizens of the Dark Dimension have millennia-long lifespans within a universe whose time passage may not match Earth’s anyway, it is almost impossible to guess how long those activities lasted. He warred with several extradimensional rulers and allied with others, forming a pact with Odin of Asgard and even joining the embodiment of Eternity to banish the occult juggernaut called Zom within a netherworldly amphora. Though not common would the clashes or direct alliances of Odin and Dormammu be in the modern-time Marvel Universe, Loki claimed that Odin spoke of Dormammu in comparison to himself, contrary to Dormammu’s belief Odin would regard him with “dread.” Dormammu’s power combined with the force of the Evil Eye was enough to nearly crush the entire Marvel Universe by merging it with the Dark Dimension, showing his potential to threaten entire realities with sufficient might. He apparently observed the Earth dimension for several millennia, supposedly even manipulating the creation of the demon Satannish in Earth’s ancient pre-history; he often clashed with Agamotto, the extradimensional entity who appointed himself that universe’s first Sorcerer Supreme and held his own against the former, and his disciple, an unnamed tentacled alien. Dormammu at some point knew of both the Celestials and Zeus, and claims he will “burn” the former while his power equals that of the latter. Dormammu also has acknowledgement and respect for the ancient Atlantean mages, even abandoning the Hood for Jennifer Kale, whom he knew has been trained by Dakimh the Enchanter himself, thousands of years later, while Kale’s Tome of Zhered-Na warns all sorcerers to reject Dormammu’s temptations, indicating even Zhered-Na and the mages of Atlantis knew of his existence and threat. The immensely powerful amulet called the Eye of Agamotto was wielded against Dormammu, as was the mysterious Staff of One of the Minorus (evokers of the Nisanti demon and thus at least indirectly his own power), but Dormammu rarely if ever met true defeat, and in time he ascended the ranks of those mystic beings whose patronage was beseeched by mortal sorcerers.
When Umar went mad after bearing a daughter to Orini, now in millennia-old adulthood, Dormammu used trickery to banish her to a pocket universe, becoming sole ruler of the Dark Dimension and continuing his conquests. Orini became Dormammu’s most devoted disciple and raised his daughter Clea in the Dread One’s service.
Finally, prompted by events that remain unrevealed (though Dormammu’s lust for further dimensional conquest to expand his own was certainly far from sated yet), Dormammu targeted the Earth dimension for conquest in 1666, causing the Great Fire of London and using the life-force of its victims to link the two dimensions via Stonehenge. However, Dormammu suffered his first defeat when forced back to the Dark Dimension by the Mystic, Earth’s Sorcerer Supreme. In 1692, Dormammu also clashed with Salem exorcist Hiram Shaw, who vied for the Sorcerer Supreme title, and other earthly foes came and went over the centuries, but the Mystic, later called the Ancient One, became his true obsession; indeed, the Ancient One’s influence spread throughout the multiverse, protecting other realms from Dormammu’s grasp. It was through the actions of the Ancient One, aided by the Eye of Agamotto, that he was deemed viable for the role of the Marvel Universe’s Sorcerer Supreme. Unable to penetrate the mystic barriers erected by his foe to protect the Earth dimension, Dormammu occasionally acted through earthly pawns over the centuries, as when, in 1888, he was involved in creating Jack the Ripper, perhaps in cooperation with mastermind Fu Manchu, via a mystic parasite later called the Zaniac.