The Holy Grail is the most famous sacred object that may never have existed as a thing at all. In the Arthurian tradition it is the vessel of the Last Supper, sometimes said also to have caught the blood of Christ at the cross, hidden away and sought by the knights of the Round Table in a quest that tests their purity. It differs from the relics of Turin, Oviedo or Valencia in kind: it is a creation of medieval romance that took on the aura of a relic, rather than a physical object with a custodial history. Precisely because the legend is so powerful, real cups have repeatedly been put forward as the genuine Grail, from the agate chalice of Valencia to the green dish of Genoa and a wooden bowl in Wales, but none has any authenticated claim. The Grail is therefore best treated as a legendary object whose importance is literary and spiritual.
Chretien de Troyes left Perceval unfinished at his death around 1190, breaking off without ever explaining what the graal actually was, part of why the object proved so adaptable to reinterpretation: Robert de Boron's Joseph d'Arimathie, written within a decade or two of Chretien's poem, supplied the Christian backstory that fixed the Grail as the vessel of the Last Supper and tied it to Joseph of Arimathea's journey to Britain, a connection with no basis in the canonical Gospels. The thirteenth-century Vulgate and Post-Vulgate prose cycles wove the Grail quest into the larger Arthurian corpus, elevating the previously minor knight Galahad into the only knight pure enough to achieve the vision. Wolfram von Eschenbach's German Parzival, composed independently, describes the Grail not as a cup but as a stone, evidence the object's physical form was never fixed within the medieval tradition itself.