Tethys and Oceanus were among the children of Gaea the Earth and Uranus the heavens. Along with their fellow Titans, they were god- like beings, but the rest of Gaea's brood were horrible monsters and Uranus banished them to the pit of Tartarus.
Gaea persuaded the Titan Cronus to slay Uranus and release her brood. He completed the first part of the pact only and Tethys, Oceanus and the others joined him in creating a paradise of subservience on the Earth.
Many years later, Cronus' son Zeus released his siblings and the brood from Tartarus, and Tethys found herself- along with the rest of the Titans of Myth- imprisoned in columns of stone in Tartarus.
After thousands of years, Thia and Hyperion escaped, and with the help of the Teen Titan Donna Troy (then Wonder Girl) they released Tethys and the others, and tried to usurp the gods of Olympus. After two epic battles during which some of the Titans of Myth were killed, Zeus invited Tethys and the remaining Titans of Myth to live in peace with the residents of Olympus and they accepted.