Merlin Ambrosius, Merddin Emrys

It was Geoffrey of Monmouth who introduced Merlin into the mythos of King Arthur. The name Myrddin is altered to Merlin to avoid a resemblance to the obscene French word merde. While Geoffrey is remembered most for his character of Arthur, it was Merlin whom he concentrated on, making the prophetic bard a central character of his three books: Prophetiae Merlini, Historiae Regum Britanniae and Vita Merlini. As a result of this second book, where Merlin appears in the tales of the king Vortigern, Aurelius Ambrosius and Uther Pendragon who reigned immediately before Arthur, Merlin in some later works also became a character in tales of Arthur.

Geoffrey tells only three tales of Merlin. In the first Geoffrey applies to Merlin the story of the boy without a father which Nennius told of Aurelius Ambrosius. Merlin is begotten on a king's daughter by a demon and the episode is now placed at Carmarthen, in Welsh Caer Myrddin. Geoffrey simply and baldly states that Merlin was also called Ambrosius to cover over his changing of Nennius. A long section of prophecy is added at this point. The second tale tells how Merlin created Stonehenge as a burial place for Aurelius Ambrosius. The third tale tells how by shape-changing magic Merlin enabled Uther Pendragon to enter into Tintagel in disguise and father his son Arthur. These episodes also appear in many later adaptations of Geoffrey's account.

Somewhat later the poet Robert de Boron retold this material in his poem Merlin with many expansions but with details garbled and changed in a way that suggests that the version of Wace, who adapted Geoffrey's account into French, had entered oral tradition and that this oral tradition was what Robert knew along with some other Merlin tales. Only a few lines of the poem have survived. But a prose retelling became popular and was later incorporated into two other romances.

In Robert's account Merlin is begotten by a devil from hell on a virgin (though she did not miraculously remain virgin) as an intended Antichrist. But his expectant mother, advised by her confessor and counsellor Blaise who realised what was amiss, had the boy baptized at birth to foil this Satanic plot. However, being half-demon, Merlin still had tremendous magical powers to know what was happening past and present and God himself gave him prophetic knowledge of the future.

Robert de Boron lays great emphasis on Merlin's power to change his shape, on his joking personality and on his connection to the Grail. This text introduces Merlin's master Blaise, who is pictured as writing Merlin's deeds which Merlin dictates to him, explaining how they came to be known and preserved. It also connects Merlin with the Holy Grail.

As the Arthurian mythos was retold and embellished upon, Merlin's prophetic aspects were sometimes de-emphasized in favor of portraying Merlin as a wizard and elder advisor to Arthur. On the other hand in Prose Lancelot it is said that Merlin was never baptized and never did any good in his life, only evil. Medieval Arthurian tales abound in inconsistancies.

In the Prose Lancelot and later accounts Merlin's eventual downfall came from his lusting after a woman named Nimue, who coaxed his magical secrets from him, eventually turning the magic he had taught her against him and imprisoning him either in a cave where he died or in a magical and invisible palace where he may live still. This was unfortunate for Arthur, depriving him of Merlin's counsel.

There are three such accounts of Merlin in Arthur's day which also cover the early days of Arthur's reign. The earliest, known as the Vulgate Merlin, includes Robert de Boron's Merlin. It was intended as a sort of prequel to the three romances of the Lancelot Cycle. An incomplete variant version known as The Book of Arthur also exists. The second is sometimes called the Huth Merlin or the Post-Vulgate Suite de Merlin or just the Suite de Merlin or Post-Vulgate Merlin. It is part of a long prose romance that has not survived intact but which is now known as The Book of the Grail or the Post-Vulgate Cycle, intended as an entire history of the Grail and of Arthur and his knights. This also includes Robert de Boron's Merlin. The third work is called The Prophecies of Merlin and contains long prophecies of Merlin (mostly concerned with thirteenth century Italian politics!), some by his ghost after his death. The prophecies are interspersed with episodes relating Merlin's deeds and with various Arthurian adventures in which Merlin does not appear at all.

The American writer Mark Twain made Merlin the villain in his novel A Connecticut Yankee in King Arthur's Court.

A modern variation of Merlin was seen in T.H. White's Arthurian retelling, The Once and Future King, in which Merlyn, as White calls him, has the curious affliction of living backwards in time to everyone else. Dan Simmons used this notion in his Hyperion, the young woman Rachel in this book suffering the "Merlin Sickness", growing progressively younger and forgetting her life day by day.

In most modern popular interpretations such as in fantasy literature and superhero stories, Merlin is depicted as a wizard of ultimate power.