1x16 - The Muppet Show

Kermit is skeptical when everyone backstage is terrorized by "the Phantom of the Muppet Show." There turns out to be one, though -- Uncle Deadly, a monster-actor who performed in the theater years ago. After being panned by the critics, he vowed never to perform there again, and not to let anyone else perform either. He warns the Muppets, "Leave or be doomed!"