Atum is the offspring of two primordial beings, born millions of years in an otherwise barren and prehistoric Earth. Gaea, also known as Mother Nature, had refused to participate with her fellow elder gods who, at this time, ran rampant over the Earth in civil war, causing death and degeneration. Gaea called upon the Demiurge, the physical manifestation of the potential life-force of Earth and source of Gaea herself, in order that it might engender within Gaea a new kind of being.

Atum arose from within Gaea fully formed and in full possession of his powers. Wielding the powers of the sun itself, Atum attacked and began to slay the elder gods, who at this time degenerated themselves into demons. As he killed them, he absorbed their essences, becoming the Demogorge. Eventually, Demogorge slew and absorbed all the demons, save for Chthon and Set, who escaped into other dimensions and tricked Demogorge into destroying lifeless replicas of themselves. Content that he had accomplished his mission, Demogorge reverted to Atum and flew upward where he merged with the sun itself.

Gaea later summoned Atum from the sun to combat the demon Set, who, determined to massacre all mammals to ensure dinosaur supremacy, had merged several enormous dinosaurs into a physical form for himself. Atum severed Set's head with a single bolt of power and absorbed the demon's power, transforming into Demogorge. Set, not deceased, began a long battle lasting months and lying waste to the Earth around them. Ironically, the dinosaurs were rendered extinct due to the battle,, and Demogorge eventually emerged victorious, driving the defeated Set into another dimension. Demogorge reverted to Atum, and was severely damaged by the battle. Atum returned to the sun, but would never again be able to come to Earth unless he was aided by other sentient beings.

In recent years, Atum was contacted by the Asgardian god of thunder, Thor, to aid Earth's heroes in a battle against the returning Set. Thor fought Atum and forced his transformation into Demogorge. As Demogorge, Atum defeated Set once again and returned to the sun.