King Arthur and his Knights by Thomas Bulfinch

The famous enchanter, Merlin, had exerted all his skill in fabricating the Round Table. Of the seats which surrounded it he had constructed thirteen, in memory of the thirteen Apostles. Twelve of these seats only could be occupied, and they only by knights of the highest fame; the thirteenth represented the seat of the traitor Judas. It remained always empty. It was called the perilous seat, ever since a rash and haughty Saracen knight had dared to place himself in it, when the earth opened and swallowed him up.

"In our great hall there stood a vacant chair, Fashion'd by Merlin ere he past away, And carven with strange figures; and in and out The figures, like a serpent, ran a scroll Of letters in a tongue no man could read. And Merlin call'd it ‘The Siege perilous,' Perilous for good and ill; ‘for there,' he said, ‘No man could sit but he should lose himself. ' " -- The Holy Grail