This is the name of the forest that Gawain rides through after he leaves the castle of the Fisher King. His path through this forest signals to the people that he has failed to ask the questions that will revive the land:

"They cannot be accused of discourtesy for that," said the knight, "for you have deserved it. They take you to be as negligent in deed as you are in speech. They saw that you had come through the Perilous Forest, and that is the road taken by all those who have failed: it is obvious from your arms and your horse." (81)
Selections from The High Book of the Grail. Translated by Nigel Bryant.