Siege Perilous

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. This knight is either Perceval or Sir Galahad, depending on the version of the story. The Siege Perilous is so strictly reserved that it is fatal to anyone else who sits in it.
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Noun
1. the legendary seat at King Arthur's Round Table reserved for the knight who would find the Holy Grail; it was fatal for anyone else to sit in it
(hypernym) seat

The seat at King Arthur’s round table that stood empty, signifying the seat of Judas. The seat remained empty until Galahad took it as a symbol of his undertaking the Grail quest. 

[Arthurian] In the Arthurian Romances, a seat at the Round Table that was kept vacant for him who should accomplish the quest of the Holy Grail. For any less a person to sit in it was fatal. At the crown of his achievement Sir Galahad took his seat in the Siege Perilous.