In Arthurian legend, the Siege Perilous (also known as The Perilous Seat) is a vacant seat at the Round Table reserved by Merlin for the knight who would one day be successful in the quest for the Holy Grail.
In Thomas Malory's Le Morte d’Arthur, in an account taken from the Vulgate Queste del Saint Graal, the newly knighted Sir Galahad takes the seat in Camelot on Whitsunday, 454 years after the death of Jesus. The Siege Perilous is so strictly reserved that it is fatal to anyone else who sits in it. Another version of this story is related in Alfred, Lord Tennyson's Idylls of the King.
Originally, this motif about the seat and the grail belonged to Percival, but the Lancelot-Grail Cycle transferred it to the new Cistercian-based hero Galahad. It appears, for example, in the earlier de Boron Didot Perceval, where Percival occupies the seat at Arthur's court at Carduel. According to many scholars, the motif of the dangerous seat can be further traced to Welsh, Cornish, and Breton mythology, where the bulk of the Arthurian legend was derived from. According to this theory, the Siege Perilous was a half-remembered version of a Celtic kingship ritual that has parallels in the Irish Lia Fail.