The Siege Perilous (or Perilous Seat) is the seat (from the French siège for seat) at King Arthur's round table in which only the chosen knight can sit. In Robert de Boron’s Estoire de Merlin, the empty seat is reminiscent of the seat that Judas vacated.
Gerbert de Montreuil, in his continuation to Chrétien de Troyes' romance Perceval states it was sent to Arthur by the fairy of Roche Menor. Six knights attempted to sit in the seat but each was swallowed by the earth. Perceval then sits in it and all six are restored to Arthur’s court. In an addendum to this story, the six knights who have suffered torments declare that the fairy that sent the chair was conveying a message that the punishments of hell will be incurred by 'those wicked souls who prefer young men to girls'.
According to the 13th century Vulgate Cycle, Uther is instructed by Merlin to build a table to commemorate the Grail Table, and at this table, an empty seat is reserved for the Grail knight. In the Queste del Saint Graal, this seat becomes known as the Feared Seat. Here, the Siege Perilous is said to parallel Christ's seat at the Last Supper and the seat of Josephus at the Grail Table. When two brothers who are jealous of Josephus object to his specially reserved seat, one of them sits in it and is destroyed. The Siege Perilous is destined for Galahad. In the Lancelot, a knight called Brumand sits in the chair only to be burned to a crisp.