Two entries with the name Perilous Valley.

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Perilous Valley

Val Perilleus

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A land ruled by the Red Knight, an enemy of Arthur. Situated on the Sea of Norway, it was ringed by high mountains and could only be entered via a narrow passage.

The Red Knight imprisoned many of Arthur’s knights in the Perilous Valley until they were freed by Gawain and Meriadeuc.

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Source
Meriadeuc or Le Chevalier aux Deux Épées | c. 1225-1250

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Perilous Valley

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A dangerous vale from which no knight ever returned alive.

Lancelot and Tristan entered the valley during the Grail Quest and slew two savage giants at the castle of Sidravalle. This might be the Perilous Valley where one of Merlin’s Stones were placed.

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Source
La Tavola Ritonda | 1325–1350

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