Eternity is Marvel comics character, created by Stan Lee and Steve Ditko. Eternity first appears in Strange Tales #138 released in 1965.
Eternity is an abstract, relatively omnipotent entity representing all time and reality in the universe. Eternity has been around before the beginning of the universe. He is eternal.
Eternity came into existence when the universe was formed (along with Galactus, Death, Oblivion, and Infinity) and spontaneously assumed the collective consciousness of all living things in the Universe. He is every living thing and every living thing is him; thus he controls everything in all plains of existence with the exception of the Living Tribunal, who maintains the cosmic balance of power. Eternity is the physical incarnation of time, whereas his sister Infinity represents space. Also, if Galactus ever dies or does not exist, the opposite of Eternity, Abraxas, would come into being.
Eternity is a being of massive cosmic power. Eternity, being an entity, exists in all places simultaneously and at all times, it merely chooses an avatar to communicate with in the physical realm. Eternity can alter space, matter, and energy around him to achieve virtually any affect he desires.