In the Arthurian romances of the Middle Ages, the character of Morgan le Fay was transformed dramatically from her Welsh original, the goddess Modron. The effect was to vilify the enchantress so that medieval Christians would not be sympathetic to her character. This study consults the oldest available Welsh mythological and historical texts as well as the medieval romances surrounding King Arthur and the Knights of the Round Table. Also consulted are some of the top contemporary Arthurian scholars. By unraveling Morgan’s transformation and the reasons for such change, it is revealed that medieval demonizing of old pagan deities was not limited to male deities. Instead, the most ancient deity of all, the Great Mother Goddess slowly became one of the most infamous characters in literature, Morgan le Fay.