The wife of the ancient being Sargon, who borrows Ann Mulhall's body and is tempted to keep it in Return to Tomorrow.
Thalassa is one of three survivors of a civilization that destroyed itself in war half a million years before Return to Tomorrow. Like her husband Sargon, she survives as pure consciousness stored in a receptacle deep within a dead planet.
The beings ask to borrow the bodies of Kirk, Spock and Ann Mulhall so they can build android forms. Thalassa takes Mulhall's body, and after feeling sensation again she is tempted to keep it, even offering McCoy a bargain and threatening him with her power. She relents, and in the end she and Sargon leave together into oblivion.