Meaning of Siege Perilous

Siege' Per'ilous

Pronunciation: [key]
— Arthurian Romance. Arthurian Romance.
  1. a vacant seat at the Round Table that could be filled only by the predestined finder of the Holy Grail and was fatal to pretenders.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.