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The Phantom of the Sewers is a crazy person who lives in the sewers below Paris and in secret rooms under a Paris opera house.
The Richie Rich/Scooby-Doo Show
He went after Shaggy Rogers, Scooby-Doo, and Scrappy-Doo when they fell into the sewers by accident. Apparently he considered them trespassers. The chase continued into rooms below an opera house and then up onto the stage, where they managed to trap him in a cage. (A Fright At The Opera)