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The Mastermind was the disguise of Holly Graham.
Powers and abilities
He had the ability to fly and teleport.
He brought back various monsters that Mystery Inc. had unmasked in the past, including the Headless Specter and the Phantom Shadow through the use of holograms. However, he was beaten by Scooby-Doo. (Scooby-Doo! Night of 100 Frights)