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The Ghoul King was the disguise of Rex Necros.
Dressed in a ripped blue and red robe, large gold crown and long grey hair, along with skeleton hands and skull, this costume used by R. Necros to become the Ghoul King.
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Scooby-Doo! Classic Creep Capers
He used it to terrify Mystery Inc. after they ruined his plans. After kidnapping the gang and taking them to an abandoned castle, Shaggy Rogers and Scooby-Doo found the other members, who set a trap for their captor. In the end, the gang solve the mystery and Necros was arrested. (Scooby-Doo! Classic Creep Capers)