This could have been the cup that held the blood of Christ, or it could be the sacred druid Regalia, depending on your religious beliefs.  The idea of the Holy Grail was something that men and women were willing to give their lives for; it is a chilling thought.  In The Mists of Avalon, the mere presence of the Holy Regalia  causes Arthur's entire court to have strange visions, and when the cup disappears, most of his court disappears with it, searching endlessly for the true Grail.
    Bradley uses the quest for the grail in her book to criticizes the mindless devotion of Christianity.  Only one who embarks on the quest meets with even partial success, and most are destroyed by there desire to find the truth, which Bradley suggests would better be found within.