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.
Perilous Valley
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. I think this is the Perilous Valley where one of Merlin’s Stones were placed.