A lake in the Forest of Morrois in Cornwall.
King Mark of Cornwall sent Sir Kay on an “adventure” to the Perilous Lake. Mark intended to ambush and kill him. Kay, unbeknownst to Mark, joined with Sir Gaheris on the way. Mark and his nephew Andred defeated Kay, but Gaheris defeated them both. Rather than kill King Mark, Gaheris forced him to revoke Tristan’s exile from Cornwall.
Despite the name, there’s no suggestion that this is magically dangerous. He puts it in the forest of Morris. Dozmary Pool, on a hilltop in Bodmin Moor, was long supposed to be bottomless, which would make it perilous. The Perilous Lake is rather minor. Malory has nothing much happening here, except that Kay and Gaheris abide at it a while waiting for King Mark.
Sources
Prose Tristan | 1230-1240
Le Morte Darthur | Sir Thomas Malory, 1469-1470