The lost civilization was a culture that lived below the surface of the planet Preplanis for over a thousand generations. They were a race of conquerors, and bred an army as an invasion force for their plan to take over the entire universe. The people were led by a young Princess who slept on a tuffett, awaiting the kiss of a visitor from another world. The home planet of whoever awoke the princess would be the first target of the invasion. (The Lost Civilization)
The lost civilization was not named in the episode.