In ancient times, a mighty Eternal roamed the Earth aiding ordinary human beings by overthrowing tyrants and slaying dangerous beasts. Human beings of those times called him by different names, sometimes mistaking him for other heroic figures. For example, it is known that the Forgotten One was mistaken for the Olympian god Hercules in ancient Greece, and, at other times, he has been known as Hero, Samson, and Gilgamesh, among others.

Eventually Zuras, leader of the Eternals, confined the Forgotten One to a sector of the Eternal city of Olympia. It is said that Zuras' reason for this action was the Forgotten One's pride and his insistence on intervening in human affairs. As yet, no details of these charges have been revealed. The Forgotten One remained banished to this sector, which the other Eternals shunned, for centuries.

Recently, while virtually all of the Eternals were meeting in the form of the Uni-Mind, the Deviants launched a spacecraft containing a powerful energy bomb in the hope of destroying the mother ship of the Fourth Host of the Celestials. Sprite, an Eternal left behind to monitor Earth, learned of the Deviants' plan and feared that the Celestials would retaliate by wreaking havoc on Earth. Sprite sought out the Forgotten One and equipped him with an armored costume and a spaceship. The Forgotten One traveled into space, and the Fourth Host's leader, the One Above All, teleported him into the Deviants' spacecraft. The Forgotten One wrecked the energy bomb and its spacecraft just before the moment of detonation. Then, floating stunned in space, the Forgotten One was drawn into the mother ship of the Fourth Host.

Some time later, the Forgotten One reappeared on Earth and was temporarily blinded (the cause of this blindness remains a mystery since Eternals cannot be injured by conventional means). Possibly the power released by the energy bomb as he wrecked it was sufficient to blind him. He delivered what he said was a message from the Celestials to the Eternals, which was to not interfere with the Celestials' plans. In recognition of the Forgotten One's bravery, Zuras restored his freedom to him and renamed him "Hero."

But the mission that the Celestials gave the Forgotten One went far beyond delivering that simple message. Exactly what that mission is remains unrevealed. However, in order to carry that mission out, the Forgotten One, once again with full sight, went to the city of the Deviants in undersea Lemuria some months later. Disguised in a hood and robes, the Forgotten One accompanied Kro, the new ruler of the Deviants, on his mission to the Pyramid of the Winds.

More recently, the Forgotten One traveled to New York, during a time of demon invasion. He joined Captain America and Thor in saving the son of Mister Fantastic and the Invisible Woman, and remained with them to assemble a version of the Avengers, whose ranks had then been depleted. He re-adopted the name Gilgamesh, at the prompting of Mister Fantastic. After several adventures with the Avengers, he sustained grave injuries in battle against the Lava Men and the extradimensional Blastaar. He was returned by the Avengers to Olympus to recuperate.

Gilgamesh was later kidnapped by the Avengers' archenemy Immortus, posing as the time-traveling Kang. As part of Immortus' ruse as Kang, he allowed Gilgamesh to be killed by the underling Newt to strike fear into the Avengers.