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The Ruined Temple is a place on Third Earth. It is an old and dilapidated temple. It is not known who built it, for whom, or when. The temple itself hides a secret. Beneath it is the prison of the goat-demon Mongor. For centuries, Mongor has been trapped in the prison, to keep him from wreaking havoc on Third Earth. In the middle of the temple, there is a giant stone slab which has ancient inscriptions and diagrams which describe Mongor, his powers, as well as the way to defeat him.
While surfing on their Spaceboards one day, WilyKit and WilyKat came across the Ruined Temple. They let their curiosity get the better of them and started to move the stone slab, freeing Mongor. Initially the ThunderKittens fled from the scene but later on, after they realized their mistake, they came back to the temple and figured out how to defeat Mongor by deciphering the symbols on the stone slab. Eventually, Mongor is defeated by Lion-O and he returns back to his prison. Panthro then seals the stone slab covering it to prevent anyone from accidentally ever opening it up again.