The Siege Perilous is the seat at the Round Table that has extraordinarily unfortunate consequences, meaning death, for anyone who sits in it who is not the chosen knight who accomplishes the quest of the Holy Grail. It was made by Merlin (a real shocker, I know) and when Sir Galahad arrived at Camelot, his name magically appeared on the chair. Sir Galahad was the purest and most honorable of all the knights, so it was only fair that he had such a special chair.

[IMAGE PENDING APPROVAL.]

Long warned of many terrors more severe
To scorch him than hell's engines could awaken,
He scanned again, too far to be so near,
The fearful seat no man had ever taken.

So many other men with older eyes
Than his to see with older sight behind them
Had known so long their one way to be wise,—
Was any other thing to do than mind them?

So many a blasting parallel had seared
Confusion on his faith, — could he but wonder
If he were mad and right, or if he feared
God's fury told in shafted flame and thunder?

There fell one day upon his eyes a light
Ethereal, and he heard no more men speaking;
He saw their shaken heads, but no long sight
Was his but for the end that he went seeking.

The end he sought was not the end; the crown
He won shall unto many still be given.
Moreover, there was reason here to frown:
No fury thundered, no flame fell from heaven.
Sir Galahad is led to the Siege Perilous when his name is inscripted on it when he arrives. (1)

http://www.lib.rochester.edu/CAMELOT/spmenu.htm
http://www.pantheon.org/articles/s/siege_perilous.html

(1) http://www.lib.rochester.edu/CAMELOT/spmenu.htm