Eternity is an abstract, virtually omnipotent entity representing all time and reality in the universe. The entity is part of a small cosmic pantheon that represents the three essential forces in the universe, being equity, necessity and vengeance.
Eternity and Infinity are in effect twin concepts, as they represent all time (Eternity) and space (Infinity) in the universe respectively. Eternity and the other cosmic beings are only surpassed by the Living Tribunal, the being that governed the Multiverse.
Eternity also spawned several "children", or concepts that became separate, independent entities. These are Empathy, Eulogy, Expediency, Entropy, Epiphany, Enmity and Eon - although the last was eventually killed and replaced in turn by the concept Epoch.
Eternity typically only manifested when there was an imminent threat to the universe, such as the villain Dormammu or the Titan Thanos, who collected the Infinity Gems and later the Heart of the Universe. With these artifacts Thanos was in fact capable of surpassing Eternity and even imprisoned the cosmic entity, proving that even Eternity was not truly omnipotent.
A notable exception was during the trial of Mr. Fantastic, when Eternity was summoned by the combined effort of Galactus and the Watcher. Eternity then allowed millions of alien onlookers to momentarily become one with the universe so that they could understand that Galactus was part of the natural order of the universe. The Sorcerer SupremeDoctor Strange was the first human being on Earth to have contact with Eternity.
Eternity and all of his counterparts across the Multiverse were guarded by their own personal Captain Universe.
Eternity was the collective consciousness of all life and was defendant on the many trillions of beings within it. It existed everywhere simultaneously. Eternity could take humanoid form when it deigned to communicate to sorcerers and the like. Eternity once aided Doctor Strange against Dormammu, and Strange then helped Eternity escape from Nightmare's clutches. He was captured by Nightmare once again, which led to the destruction of Earth, but at the intervention of Strange and the Ancient One, Eternity recreated the Earth. To have greater understanding of humanity, Eternity occasionally walked the Earth, using the name Adam Quadmon.
When Henry Pym grew beyond the macroverse into Overspace, he met Eternity, who thanked him for saving reality from Chthon. He gave the title "Scientist Supreme" to Pym because of his desire to take science to the point of studying magic. He sent Pym back to Earth, telling him that he was the founder of the Avengers to come and promised a new age of heroes was coming, although this may have been a ruse by Loki.
The Eternity of Earth-616, as well as its alternate reality counterparts, was annihilated by the Beyonders.
Omnipotence. Unlimited ability to manipulate time, space, matter, energy, magic or reality for any purpose; Immortal embodiment of the universe; Capable of physically manifesting as any force living within the Earth dimension