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The Cuarto Llave is Spanish for "Fourth Key". It is in the form of a torn piece of sailcloth. The item was originally used by the Benevolent Lodge of Mystery, a mystery solving group in the 1800s.
Scooby-Doo! Mystery Incoporated
Scooby-Doo found it in a book while they were looking in the Crystal Cove Library for clues to the Dandy Highwayman.
It appeared floating in the air in the time-frozen scene of the Benevolent Lodge of Mystery at the Burlington Library found in the sitting room. (Nightmare in Red)
The gang packed it in a bag and brought it along on their mission to find and destroy the Evil Entity.
It was vitally important for blocking a wind trap at the door to the Air Dimension.
- Like the other keys, the frozen scene associates it with a Greek element, in this case air.