Alright, that's it. Time to save Merlin from the wretches of fantasy fiction.
from the Welsh Myrddin, "man from Carmarthen."
Carmarthen's name in Welsh is Caerfyrddin, translated as "Camp Myrddin." The name derives from the old Roman fort of that name, Maridunum, meaning, the Fort by the Sea--"mari" from "mara"--sea in Latin, and "dunum" from "dun"--fort in the Old Celtic languages. It was common for the mixing of Latin and Celtic languages in naming places in Britain, as Latin and Celtic were very similar in their vocabularies.
When Geoffrey of Monmouth decided to write his History of the Kings of Britain, he combined a figure named Myrddin, who was a sixth and seventh century bard and prince in northern Wales and southern Scotland who went mad during the battle of Arthuret, with a figure named Ambrosius, a boy spawned by a demon gifted with prophecy. According to Nennius' history, Ambrosius was sent for by Vortigern as a sacrifice to the gods so that he could build a stable tower at Dinas Emraise, sometmimes called Dinas Emrys, which, btw, is also the Welsh spelling for Ambrosius--Emrys. Ambrosius was able to escape being sacrificed by telling Vortigern that there were two dragons buried under the hill, two dragons that had been fighting for centuries--the Red Dragon and the White. Vortigern let them out, and they destroyed each other, the Red killing the White, but losing its life in the process. Ambrose explained the the Red Dragon was the British, and specifically, a British leader, and the White Dragon was the Saxon invaders, with whom Vortigern had certain dealings. The Britians would be victorius against the Saxons, but only for so long, and their leader would die at their hands. This is later applied to Arthur, who put a red dragon on his banner, and the Red Dragon of Wales is now seen on the Welsh flag.
The bard Myrddin has several poems attributed to him in The Black Book of Carmarthen (appropriate, huh?) and The Red Book of Hergest, the later of which is also the source for the Mabinogion; curiously, while no stories about Myrddin appear in the Mabinogion, the story of the concealment of the two dragons appears in the story "Cyfranc Lludd a Llefelys." The poem "Armes Prydein Vawr"--"The Greater Prophecy of Britain"--appears in the Lyfyr Taliesin--the Book of Taliesin, but Taliesin attributes it to "Merdin." This Myrddin was the son of Madog Morfryn, the son of Morydd, the son of Mor, the son of Ceneu, the son of Coel, as in Old King Cole, and thus was a member of British royalty. His sister Gwenddydd married Rhydderch Hael, a king of Strathclyde who fought at the battle of Arthuret (c. 575 CE), wherein Peredur/Perceval, Rhydderch Hael, Aedan mac Gabhran and Maelgwn Gwynedd fought with Gwenddolou the pagan king. Myrddin went insane from this battle, presumably because he was on the losing side, which was the side against all his friends.
So Geoffrey combines these two figures into one Merlin. He changed the name because if he were to Latinize the name Myrddin, it would be spelled Merdin--which looks too much like "merde" to his French-speaking patrons. (Merde, btw, means "shit".) Instead, he substituted the "d" with an "l." In Geoffrey's book, Merlin is advisor to Uther Pendragon; he has Uther shapeshift so that the king will sleep with Igrain and beget King Arthur. Strangly, he then suddenly disappears from the book until the very end, where Geoffrey says that Merlin prophecied that Arthur wouldn't come again until nearly the end of the world.
Geoffrey is attributed with a book called the Vita Merlini, or Life of Merlin, which tells of Merlin's years after Arthur has died. It relies on the myth of the Myrddin who went mad at the battle of Arthuret, and thus on Welsh legend.
From here, Merlin goes on to intruge all sorts of medieval romancers. Robert de Boron dedicates a whole third of his Le Roman du Graal to his story, which states that Merlin is the son of the Devil and supposed to become the Antichrist, but since he was baptised, he now only practices white magic. He gets Uther and Ygrain together, and takes Arthur to be raised.
He sets up the Round Table, puts the sword in the stone, brings in Guenevere, and councils Gawain and Perceval to start the Quest for the Holy Grail. This is Merlin's origin as the wizard we picture today.
Later romancers used this idea of the redeemed Antichrist and his doomed relationship with Vivian/Nimue, until crystalized by Sir Thomas Malory's epic La Morte d'Arthur in 1470. Here, Merlin is the famous wizard who engenders Arthur's birth and raising by Sir Ector, puts the sword in the stone, and the rest already added by de Boron.
Later writers would also have their way with Merlin. He became a useful name to attatch to any ridiculous prophecy in the almanacs of the time, which is what prompts this sequence from Shakespeare's King Lear, as spoken by the Fool:
This is a brave night to cool a courtezan.
I'll speak a prophecy ere I go:
When priests are more in word than matter;
When brewers mar their malt with water;
When nobles are their tailors' tutors;
No heretics burn'd, but wenches' suitors;
When every case in law is right;
No squire in debt, nor no poor knight;
When slanders do not live in tongues;
Nor cutpurses come not to throngs;
When usurers tell their gold i' the field;
And bawds and whores do churches build;
Then shall the realm of Albion
Come to great confusion:
Then comes the time, who lives to see't,
That going shall be used with feet.
This prophecy Merlin shall make; for I live before his time.
--as Lear/Llyr lived long before the time of King Arthur and Merlin.
Shakespeare is sometimes (spuriously) connected to a play about Merlin called The Birth of Merlin, or, the Childe Hath Found His Father. Johnathan Swift wrote satirical "prophecies" of Merlin, and Mark Twain used Merlin as a satire of Alfred Lord Tennyson in the book A Conneticut Yankee in King Arthur's Court.
Merlin lives on in the modern era as archetype (Obi-wan Kenobi), comedic relief (The Sword in the Stone the film, not the book; Excalibur), and the generic wise old man. Some try to link him to the druids, though this cannont be proven, while others hold him to be a bard. Some say he was Taliesin, an idea based on a poem attributed to the bard but of a much later date:
Primary bard to Elphin am I
And my country is the kingdom of the summer stars
Iddo and Heinin know me as Merddin (Myrddin/Merlin)
But at lenght, all men will call me Taliesin.
A twelfth-thirteenth century lost romance by Robert de Boron, part two of his Le Roman du Graal, a Grail Romance written after Chretien's (but possibly without knowledge of Chretien's).
The Romance borrows from Geoffrey of Monmouth's History of the Kings of Britain, with the details of Merlin's place as son of the devil, the story of the dragons, and the changing of Utherpendragon. However, it also adds the detail of the Sword in the Stone.
It also evidently borrows from Geoffrey's The Life of Merlin, as de Boron's romance includes the story of Merlin's triple prophecy of death, which is also found in the Lailoken tales, in which it is assumed Lailoken is Merlin. The story goes like this: a king challenges Merlin's abilities of prophecy. He devises a plan to trick Merlin into prophecying the death of the same individual (usually the king, but in one version that of Merlin himself) three times. Each time Merlin gives a different prophecy, usually following these lines: that the person will be hung; that the person will drown; that the person will break his neck. The king disbelieves Merlin, until the individual in question dies, usually falling from a bridge, wherein
its rider plunged down the steep cliff slope into the river. But he fell in such a way that one foot caught in a tree and the rest of his body was submerged in the flowing stream. So then, he fell-he was drowned-he hung from a tree; and by his triple death he proved the prophet a true one.
The Life of Merlin