With half a blue face, at times a whole face. Omniscient and first force of reality. Eternity can transform himself from the fabric of the universe into a figure form. He also has many cosmic powers, which Adam Warlock may have stole some from. It's just simple that if he or the other forces of reality, Galactus, or Death die, all life will stop and cease.
Eternity is just that, and is a force of reality.
As the first force of reality, followed by Galactus, then Death, if anyone is strong enough to destroy any of these three beings, all life stops and cease. He is a Cosmic Entity like others. Galactus, Lord Chaos, Master Order, and Death are also Cosmic Entities. Later, he actually met Galactus, after a group of Elders tried to destroy Galactus. If it wasn't for Silver Surfer intervention, the Kree race would destroy Galactus, but Surfer obtained the Soul Gem and give it to the Elders.