Reference

The Siege (from the French siège, seat) Perilous is the seat at Arthur's Round Table in which only the chosen knight can sit.In the prose rendition of Robert de Boron's Merlin, the empty seat is reminiscent of the seat that Judas vacated.There is also an empty seat at Joseph of Arimathea's Grail Table which destroys anyone unworthy of sitting there.

The Siege Perilous: A Seat for the Chosen. The Siege Perilous, the perilous seat at Arthur's Round Table, was reserved for the knight who would seek the Holy Grail. Many tried to claim it but met untimely fates. However, upon Galahad's arrival at Camelot, the seat miraculously bore his name, marking him as the chosen one.

Sir Galahad takes the Siege Perilous at the Round Table, in a 15th-century illustration. In Arthurian legend, the Siege Perilous (Welsh: Gwarchae Peryglus, also known as The Perilous Seat, Welsh: Sedd Peryglus) 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. [1]