First Appearance: Eternals v1 #1 (July 1976).
Appearances: Too many others to list.
Years Active: circa 1,000,000 B.C.E.-present.
The Celestials, a group of cosmic gods whose purpose is to judge the fitness of other races, visited the Earth at the dawn of humanity, circa 1,000,000 B.C.E. At this time the Celestials created a race of super-powered beings designed to serve the Celestials, to aid them during the time of judgment (when the Celestials would decide whether whatever races evolved on Earth were worthy), and then assist them when the Celestials winnowed out the races on Earth (since the Celestials never judged any other race worthy).
This race of super-powered beings was the Eternals. The Celestials left soon after they created the Eternals, leaving the Eternals behind to further their plans on Earth. The Eternals did this during the long millennia in which the Celestials were gone, and did this during the few occasions in which the Celestials returned to Earth to see how things had progressed. The Eternals also fought against the enemies of the Celestials, such as the Deviants. Recently, when the Celestials finally judged humanity worth keeping, most of the Eternals left for space to accompany the Celestials on their next judgment. Only a few Eternals remained behind.
The Eternals have been around, in hiding, for all of human history. Only a few of them, however, like Makkari and Gilgamesh, were active in the affairs of mankind.
The Eternals are very powerful, with superstrength, flight, and energy manipulation being their main powers.
Useful Eternals Sites
Sersi's Loft A very good site on the Eternals.