The transformation of Joseph from a quiet Gospel figure into a hero of romance took place in the twelfth and thirteenth centuries, in the literature of the Holy Grail. The Grail enters written record in the unfinished Perceval of Chretien de Troyes around 1190 as a mysterious object; it was Robert de Boron, shortly after, who fixed its Christian meaning and tied it to Joseph.
In Robert's Joseph d'Arimathie, the vessel is the cup of the Last Supper, in which Joseph is said to have caught the blood of the crucified Christ. Imprisoned and sustained miraculously by the Grail, Joseph is later freed, gathers followers, and leads them westward to Britain, carrying the sacred cup. On this framework the vast later cycle of Grail romance was built, and Joseph became the ancestor of the Grail's guardians.
The atlas relates Joseph to the Holy Grail on the explicit understanding that this is a medieval literary development, not a historical claim: no physical object has ever been authenticated as the Grail, and its very existence as a relic is unestablished. The Grail entry is classed as legendary, and Joseph's connection to it is a debated relation, a figure of the Gospels drawn, a millennium later, into a story the Gospels never told.