The opening tale of the Morte Darthur, “Tale of King Arthur,” is based on the Prose Merlin and its Post-Vulgate Suite. The Prose Merlin is the redaction of Robert de Boron's poem of the same name, of which only the first 504 lines are extant. This poem is preceded in the sole surviving manuscript by Robert's Joseph d'Arimathie or Roman de l'Estoire dou Graal, so that together they tell the story of the Grail from the time of Christ to its voyage to Britain and begin the history of Arthurian time with the events leading to the birth of Merlin. Scholars believe that this Joseph and the Merlin fragment are the remains of an Arthurian poetic cycle that predates the prose cycles but which survives in a prose redaction. This redaction contains a prose rendering of the Joseph, and the Prose Merlin, which advances the story to Arthur's coronation, and concludes with a Prose Perceval, once commonly called the Didot Perceval, which follows the successful Grail quest with a brief account of the destruction of Arthur's realm.

With minor alterations, the Prose Merlin was made into a part of both the Vulgate and Post-Vulgate cycles, each cycle following Arthur's coronation with a different series of adventures. The Vulgate Suite du Merlin concentrates on large-scale battles between Arthur's forces and rebel kings, invading Saxons, and ultimately the Roman Empire, the defeat of which makes the climax of the Vulgate Suite.