Siege Perilous
In Arthurian legend , the Siege Perilous ( , 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 . 
<triplet> Round Table <subj> Arthurian legend <obj> present in work