Siege Perilous

In Arthurian Legend, the Siege Perilous was a specially reserved seat at the Round Table which was kept vacant by Merlin for the knight who was destined to quest for and return with the Holy Grail. Sir Galahad was the purest, and most honorable of all the knights of the round table and he alone completed the quest and earned the Grail. The Siege Perilous was so strictly reserved, that it was fatal to anyone else who sat in it.

After being knighted by his father Sir Launcelot, Galahad was led to his seat in Camelot on Whitsunday, 454 years after the death of Jesus Christ.

An account of this is recorded in Thomas Malory's "Le Morte D'Arthur" and in Alfred Lord Tennyson's "Idylls of the King".