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.[1] […] The Siege Perilous is so strictly reserved that it is fatal to anyone else who sits in it.
[…] According to many scholars, the motif of the dangerous seat can be further traced to Welsh, Cornish, and Breton mythology, whence the bulk of the Arthurian legend was derived.
Source: Siege Perilous – Wikipedia
Tim B.
“In the Lancelot, a knight named Brumand, trying to perform an act that Lancelot never dared to do, sits in it and is burned to a crisp. Malory says that Merlin made the Siege Perilous for the greatest Grail knight. When Galahad arrives at Camelot, his name appears on the seat destined for him.”
https://d.lib.rochester.edu/camelot/theme/siege-perilous