Between 1200 and 1210 the German writer Wolfram von Eschenbach composed an epic romantic poem, tens of thousands of lines long, called Parzival. It drew on the hugely popular legends of King Arthur, which had for decades delighted aristocratic audiences with tales of chivalry and questing, love and betrayal, magic and combat.

Eschenbach’s patron was one Hermann, Landgrave of Thurangia, but the readership his work eventually found was enormous and its influence immense. More than 80 medieval manuscripts of the poem still survive.

In Parzival, the eponymous young hero appears at Arthur’s court and quickly becomes embroiled in a dispute with a “red knight,” whom he kills in a fight. After going away to learn to be more chivalrous, Parzival embarks on a search for something called the Grail: both a literal hunt for a mysterious, life-giving stone and a spiritual journey toward enlightenment in God. The Grail is initially guarded in a magical castle by a character called the Fisher King, who is in constant pain from a wound to his leg, divine punishment for his failure to remain chaste.

Parzival meets the Fisher King, then becomes sidetracked by other escapades. Eventually, after fighting a knight who turns out to be his own brother, he learns that he himself has become the new king destined to guard the Grail, and the story eventually draws to a close.

Much of Parzival, as written by Eschenbach, was unoriginal. Earlier writers such as Geoffrey of Monmouth and Chrétien de Troyes had already established the world he described and provided many of the plots.

Eschenbach was essentially updating: expanding the characters’ adventures and giving the tales a flavor that he thought his own readers (and listeners, for many would have heard the poems read aloud in their lords’ halls) would enjoy.

One of the ingredients he added was the appearance of a military order called the Templeise, warriors sworn to chastity who help the Fisher King keep watch in their “temple” over the Holy Grail. These men were not identical to Templars: Their symbol was a turtledove rather than the crusaders’ cross, and they did not appear to have a developed rule.

All the same, the resemblance was striking and the story proved to be enduring. The Templars had been transformed for the first time from a crusader militia into the guardians of the mythical Grail.

During the later Middle Ages the legend of the Grail became ever-more developed and fanciful. In the stories by Chrétien de Troyes and Wolfram von Eschenbach, the Grail was a somewhat indistinct object—a plate, a stone, a bleeding lance or a goblet—all of which served ultimately as a metaphor for Jerusalem and for the kingdom of Heaven itself.

But as time went on, writers began to imagine the Grail to be something quite specific. Robert of Boron, writing shortly after Eschenbach, concocted a backstory for the Grail, in which it was both the cup used by Christ at the Last Supper and the vessel used by Joseph of Arimathea to collect drops of Jesus’s blood as he hung dying on the cross. It was now a Holy Grail. In this telling, the Holy Grail was then transported by Joseph from the Holy Land to Avalon, which came to be identified with a specific location: Glastonbury, in the southwest of the British Isles.

All of this was high fantasy, which people in the Middle Ages would have recognized as such: The Holy Grail was no more real than Spectre in today’s James Bond films. Yet as time went on, Robert of Boron’s entertaining but bogus pseudo-history became confused with the real history of Christianity, and of crusading.

Today, writers of fiction—and even of history—often assume that the Holy Grail was a real object, and combine its supposed presence in Jerusalem with the story of the Templars, speculating that one of the order’s first purposes in the Holy Land was to protect the cup of Christ. From this exciting but illusory point of departure, it’s possible to speculate that the “real” reason the Templars were destroyed by Philip IV of France was connected to some conspiracy to strip the brothers of their most holy secrets.

Ironically, some modern writers, beginning from the false assumption that the Holy Grail was a real object, have gone on to argue that in fact the Grail was actually a metaphor! Much play has been made of the pun in medieval French on san graal (holy grail) and sang real (royal blood). Perhaps, runs this argument, the grail was not a mystical vessel after all, but code for a secret line of descent from Jesus and Mary Magdalene.

This is all great fun, but it belongs to the realm of fantasy. As we have seen, the Grail was never a real physical object. It began as a literary motif and an allegory, both for crusading in general and for the individual quest for salvation. Philip IV, for his part, had far better reasons for destroying the order of the Temple than retrieving from them an imaginary goblet.

But the Holy Grail myth is a brilliant starting point for fiction, whether that means re-telling the Arthurian legends, or inventing stories about the deeds of the Templars—or a fusion of the two. Many historians become irritated by the conflation of fantasy and fact, but there is a huge popular appetite for tales of the Grail, particularly when the Templars, endlessly fascinating, are included.

That was undoubtedly the case in the early 13th century when Wolfram von Eschenbach and Robert of Boron were writing their epic romances. Today, around 800 years on, it is as true as ever.