In the face of modern depictions of Merlin as everything from comic sorcerer complete with pointy hat to proto-environmentalist Druid, medievalists – particularly those who teach undergraduates – often find themselves emphasizing him as first and foremost a prophet in the eyes of medieval audiences. Geoffrey of Monmouth, the first main promulgator of the matter of Merlin in writing, claims that interest in Merlin's prophecies was so great that he had to stop work on the Historia to translate them. Geoffrey interrupts his narrative with an unusual first-person comment:

In the Historia, when the doomed usurper Vortiger has haled Merlin into his court, after being advised to temper the mortar of his castle's unstable foundation with the blood of someone who has no father, Merlin reveals that the foundation is undermined by a pool of water inhabited by two dragons, red and white, the sight of which inspires him to deliver a lengthy, violent, cryptic beast allegory extending well beyond the meaning of the dragons themselves. As Crick has argued, the presence of prophecy seems only to have strengthened the authority of the entire Historia for much of its audience: ‘Merlin seems generally to have enhanced, not detracted from, Geoffrey's respectability as a historian.’ Geoffrey's prophecies, which constitute Book 7 of the Historia, were also copied and transmitted separately. New prophecies were being composed and attributed to Merlin into at least the fourteenth century, and they remained common enough cultural currency that Shakespeare can have the Fool joke about them, prophesying prophecy, in King Lear.