An immensely advanced humanoid civilisation whose world Arret was laid waste by war half a million years ago, leaving three survivors, Sargon, Thalassa and Henoch, preserved as pure minds inside glowing receptacles.
The Arretans appear in the Original Series episode Return to Tomorrow. Half a million years before the 23rd century they had spread colonies among the stars, and Sargon suggests that some humanoid peoples may be descended from them. Their powers grew until, as he puts it, they dared to think of themselves as gods, and a final war tore away their planet's atmosphere.
A few of the best minds from both sides were stored in spheres deep underground. By 2268 only three still function: Sargon, his wife Thalassa and Henoch, a former enemy.
They ask to borrow the bodies of Kirk, Ann Mulhall and Spock long enough to build android forms. Henoch, occupying Spock, schemes to keep the body and kill Sargon. He is tricked into fleeing it and is destroyed. Sargon and Thalassa decide the temptation is too great and choose to depart into oblivion together.