Soon after Earth was formed, the entity the Demiurge materialises, being the physical manifestation of the life that would eventually manifest on the planet. The Demiurge seeds the Earth with fragments of its being, which in turn give rise to the Elder Gods. This ancient pantheon includes Chthon, Set and Gaea, who will one day be the mother of the Thunder God Thor. Gaea immediately begins to nurture the more delicate forms of life that are slowly emerging on Earth, while the other Elders Gods grow restless and agitated. Millions of years later, Set discovers murder and slaughters one of his brethren. This sparks a full-scale war, and the Elder Gods gradually devolve to match their new-found hostilities, becoming demons.
Fearful for the new life that could be destroyed in the demonic war, Gaea summons the Demiurge, who impregnates her with part of its essence. Gaea later gives birth to Atum - a golden humanoid imbued with the power of the Sun itself - that emerges and attacks the Elder Gods. As Atum kills each one and absorbs their life force, he is changed by their evil energies and devolves into a huge, hulking demonic being with wings and claws - Demogorge, the God Eater. The God Eater almost kills them all: only Chthon and Set survive by fleeing into alternate dimensions. With Gaea the only Elder God remaining, the God Eater sheds the Elder Gods' energies and becomes Atum once again. Atum then journeys to the Sun and hibernates there until needed once more.  Thousands of years later, a group of eight Death Gods from various pantheons (including Hela; Pluto; Seth and non-Death God Mephisto) combine their mystical might to join all the Hells into one vast dimension. This act rocks the cosmic axis, and in doing so forces the intervention of the God Eater. The Death Gods prove to be no match for Demogorge, and all but the fleeing Hela are consumed. Alerted to the act and presence of the God Eater by his faithful ravens, Odin takes counsel with the other heads of the pantheons - the Skyfathers - who each sent a champion to stop Demogorge and prevent further disaster. Led by Thor (coincidentally Atum's half-brother, since Gaia is also his mother), the champions (who included the Olympian Apollo and the Heliopolitan Horus) find the God Eater just as it is consuming Hela.
The champion gods battle and wound Demogorge, but it still manages to consume them all with the exception of Thor. Thor realizes that while the God Eater is impervious to almost all external attacks, it may be able to be damaged internally. Plunging into one of Demogorge's orifices, Thor attacks the God Eater's inner workings. Damaged beyond repair, the God Eater can no longer contain the energies it has consumed and releases all the previously consumed gods, and restores the Hells to their rightful dimensions.
When the Elder God Set is summoned back to Earth by the Deviant villain Ghaur, Thor travels to the Sun and forces the intervention of Atum, who becomes Demogorge the God Eater. Demogorge then consumes Thor in a fit of rage, but is subsequently taken over by Thor, who uses the God Eater's powers to defeat Set. A repentant Atum then releases Thor and returns to the Sun. 
When the alien Skrulls invade Earth at the behest of their deities, a cadre of gods is formed to combat Kly'bn and Sl'gur't, the Skrull gods. After being contacted by his descendant Horus, Atum joins the Earthly pantheon, which consists of Hercules; Snowbird; Amatsu-Mikaboshi and Ajak.  During the confrontation, Atum is killed after trying to devour Sl'gur't. Atum's spine is later used by Snowbird to kill the god Kl'byn. 
Atum possesses the power of the Sun itself, and can project intense forms of both heat and light. Atum also possesses the power of flight. When fighting gods, Atum quickly reverts to God Eater form, and while possessing incredible strength, stamina, and flight in this form, it is not totally invulnerable as it has been hurt by divine weapons of exceptional quality. The God Eater's most disturbing trait is its ability to convert gods into energy and absorb them through orifices in the palms of its hands. During this act the rapidly dissolving god's form can be seen coursing through the God Eater's veins. A god consumed by the God Eater retains some shape and sentience and manifests as a face on the entity's body.