Giles: It's the vampire equivalent of the Holy Grail.
Legend has it that the Holy Grail is the cup that Jesus drank from during the Last Supper, and that it was also used to catch some of his blood when he was dying on the cross. This cup became an object of great reverence, but was lost in the very early days of the Christian Church. For centuries people have spent lifetimes searching for the Grail including, most famously, the knights of King Arthur's round table. The term "Grail" or "Holy Grail" are now used to mean something that is searched for but is not real or, at least, is never found.