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 then returned to the sun.
Atum possesses various superhuman abilities that are common to the various god pantheons of Earth. However, he is considerably more powerful than the vast majority of them.
Absorption and Transformation: Although Atum is already immensely powerful, he has the ability to literally absorb and consume other gods as well as demons. As a result, Atum's body basically feeds upon these absorbed beings like food and, as a result, their powers are added to his, thereby increasing all of his powers beyond their limits. However, if he absorbs too much energy, he transforms into the Demogorge, which is a hulking, almost unstoppable brute. After Atum has purged himself of this excess energy, which he can do at will by releasing the beings he absorbs or burns the energy away in combat, he will revert to his normal form.
Superhuman Strength: Atum possesses vast superhuman strength, the limits of which aren't known. However, it is believed that his physical strength surpasses that of any member of the god or demon pantheons of Earth. Hence, Atum is ultimately able to lift far in excess of 100 tons.
Superhuman Speed: Atum is capable of running and moving at speeds that are beyond the physical limits of even the finest human being, or most other gods for that matter.
Superhuman Stamina: Atum's advanced musculature produces no fatigue toxins during physical activity, granting him limitless superhuman stamina.
Superhuman Durability: Atum is highly resistant to all forms of physical injury. Even without transforming into the Demogurge, Atum can withstand attacks from some of the most powerful of Earth's gods and demons without being injured. Atum can fully and easily survive the temperature and pressure within a star and the vacuum of deep space indefinitely.
Superhuman Agility: Atum's agility, balance, and bodily coordination are enhanced to levels that are beyond the natural physical limits of even the finest human athlete.
Superhuman Reflexes: Atum's reflexes are similarly enhanced and are superior to those of the finest human athlete.
Regenerative Healing Factor: Although he is virtually invulnerable, it is possible for Atum to suffer physical injury. However, his body is capable of rapidly regenerating damaged or destroyed bodily tissue faster and more extensively than most other gods, even to the point of easily regrowing lost limbs.
Immortality: Atum, like some of the god pantheons on Earth, is functionally immortal in the sense that he is completely immune to the effects of aging. He is also immune to all Earthly diseases and infections.
Flight: Atum, even while transformed into the Demogurge, is capable of flying through the air, or even through deep space. The upper limit of his flight speed isn't revealed, though he can achieve faster than light speeds.