Geoffrey's rendering of the character was immediately popular, especially in Wales. Later writers expanded the account to produce a fuller image of the wizard. Merlin's traditional biography casts him as a cambion: born of a mortal woman, sired by an incubus, the non-human wellspring from whom he inherits his supernatural powers and abilities. The name of Merlin's mother is not usually stated but is given as Adhan in the oldest version of the Prose Brut. Merlin matures to an ascendant sagehood and engineers the birth of Arthur through magic and intrigue. Later authors have Merlin serve as the king's advisor until he is bewitched and imprisoned by the Lady of the Lake.
Later adaptations of the legend
Robert de Boron lays great emphasis on Merlin's power to shapeshift, on his joking personality, and on his connection to the Holy Grail. This text introduces Merlin's master Blaise, who is pictured as writing down Merlin's deeds, explaining how they came to be known and preserved. Robert was inspired by Wace's Roman de Brut, an Anglo-Norman adaptation of Geoffrey's Historia. Robert's poem was rewritten in prose in the 12th century as the Estoire de Merlin, also called the Vulgate or Prose Merlin. It was originally attached to a cycle of prose versions of Robert's poems, which tells the story of the Holy Grail: brought from the Middle East to Britain by followers of Joseph of Arimathea, the Grail is eventually recovered by Arthur's knight Percival.
The Prose Merlin contains many instances of Merlin's shapeshifting. He appears as a woodcutter with an axe about his neck, big shoes, a torn coat, bristly hair, and a large beard. He is later found in the forest of Northumberland by a follower of Uther's disguised as an ugly man and tending a great herd of beasts. He then appears first as a handsome man and then as a beautiful boy. Years later, he approaches Arthur disguised as a peasant wearing leather boots, a wool coat, a hood, and a belt of knotted sheepskin. He is described as tall, black and bristly, and as seeming cruel and fierce. Finally, he appears as an old man with a long beard, short and hunchbacked, in an old torn woolen coat, who carries a club and drives a multitude of beasts before him (Loomis, 1927).
The Prose Merlin later came to serve as a sort of prequel to the vast Lancelot-Grail, also known as the Vulgate Cycle. The authors of that work expanded it with the Vulgate Suite du Merlin (Vulgate Merlin Continuation), which describes King Arthur's early adventures. The Prose Merlin was also used as a prequel to the later Post-Vulgate Cycle, the authors of which added their own continuation, the Huth Merlin or Post-Vulgate Suite du Merlin.
In the Livre d'Artus, Merlin enters Rome in the form of a huge stag with a white fore-foot. He bursts into the presence of Julius Caesar and tells the emperor that only the wild man of the woods can interpret the dream that has been troubling him. Later, he returns in the form of a black, shaggy man, barefoot, with a torn coat. In another episode, he decides to do something that will be spoken of forever. Going into the forest of Brocéliande, he transforms himself into a herdsman carrying a club and wearing a wolf-skin and leggings. He is large, bent, black, lean, hairy and old, and his ears hang down to his waist. His head is as big as a buffalo's, his hair is down to his waist, he has a hump on his back, his feet and hands are backwards, he's hideous, and is over 18 feet tall. By his arts, he calls a herd of deer to come and graze around him (Loomis, 1927).
These works were adapted and translated into several other languages. The Post-Vulgate Suite was the inspiration for the early parts of Sir Thomas Malory's English language Le Morte d'Arthur. Many later medieval works also deal with the Merlin legend. The Italian The Prophecies of Merlin contains long prophecies of Merlin (mostly concerned with 13th-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 earliest English verse romance concerning Merlin is Arthour and Merlin, which drew from the chronicles and the French Lancelot-Grail.
As the Arthurian myths were retold and embellished, Merlin's prophetic aspects were sometimes de-emphasised in favour of portraying him as a wizard and elder advisor to Arthur. On the other hand, in the Lancelot-Grail it is said that Merlin was never baptized and never did any good in his life, only evil. Medieval Arthurian tales abound in inconsistencies.
A manuscript found in Bath from the 1420s simply records a "Merlyn" as having helped Uther Pendragon with his "sotelness" or subtleness, presumably but not necessarily magic. His role could be embellished and added to that of Aurelianus Ambrosius, or he could be made into one of old Uther's favourite advisors and naught more.
When Niviane finally goes back to her country, Merlin escorts her. However, along the way, Merlin receives a vision that Arthur is in need of assistance against the schemes of Morgan le Fay. Niviane and Merlin rush back to Arthur's castle, but have to stop for the night in a stone chamber, once inhabited by two lovers. Merlin relates that when the lovers died, they were placed in a magic tomb within a room in the chamber. That night, while Merlin is asleep, Niviane, still disgusted with Merlin's desire for her, as well as his demonic heritage, casts a spell over him and places him in the magic tomb so that he can never escape, thus causing his death.
Merlin's death is recounted differently in other versions of the narrative; the enchanted prison is variously described as a cave (in the Lancelot-Grail), a large rock (in Le Morte d'Arthur), an invisible tower, or a tree. In his book "The Meaning of Trees: botany, history, healing, lore" Fred Hageneder writes on page 149, "According to Breton legend, the legendary wise man Merlin climbed the Pine of Barenton (from bel nemeton, "Sacred Grove of Bel"), just as shamans climb the World Tree. Here, he had a profound revelation and he never returned to the mortal world. In later versions,Merlins glas tann was mistranslated as a "glass house". It is actually a living tree (from the Cornish glas "(ever)green", and tann, "sacred tree"), and from these words the name of Glastonbury, in Somerset, England is sometimes derived. Hence, according to legend, it is a sacred tree in which the soul of Merlin awaits his return." In the Prophetiae Merlini, Niviane confines him in the forest of Brocéliande with walls of air, visible as mist to others but as a beautiful tower to him (Loomis, 1927). This is unfortunate for Arthur, who has lost his greatest counselor. Another version has it that Merlin angers Arthur to the point where he beheads, cuts in half, burns, and curses Merlin.