Medieval depiction of the story of Merlin and the two dragons related by Geoffrey of Monmouth.
Merlin is mentioned in a few surviving Dark Age manuscripts, he is only referenced in passing. The first author to provide any actual detail concerning Merlin's life was the Welsh cleric Geoffrey of Monmouth who wrote in the 1130s. In his History of the British Kings Geoffrey introduces Merlin by saying that he first proved himself as a youth when a British king named Vortigern chose him as a sacrifice. According to Geoffrey, Vortigern was building a fort on a mountain in North Wales to protect his kingdom from the invading Anglo-Saxons, but each time the fort was close to completion the foundations mysteriously collapsed. Vortigern's advisors suggest that to put things right a boy must be sacrificed, and victim they pick is the young Merlin. However, just as Merlin is about to die, he tells the king that the problems are being caused by two dragons that dwell in a pool, in a cave below the fort's foundations. When the pool is discovered and the dragons released, Vortigern is so impressed by Merlin's mystic knowledge that he makes him his chief advisor and offers him the new fort as his own. Although this story is obviously an imaginative legend, a Dark Age manuscript records a similar story which reveals an historical figure behind the Merlin myth.
Vortigern was certainly an historical figure: he is recorded by Dark Age writers as the ruler of much of Britain in the mid-fifth century. Around the year 830 one Dark Age chronicler refers to King Vortigern when recounting a similar legend to Geoffrey's story of Merlin and the two dragons. In his History of the Britons the British monk Nennius wrote about a young man who saved himself being sacrificed by Vortigern by revealing that two dragons dwelt in a cave below the king's fort. The story is almost word-for-word that told by Geoffrey; the only difference is that the youth is not called Merlin, but Ambrosius. As both accounts are virtually identical, could this Ambrosius have been the man upon whom the stories of Merlin were based?
Vortigern escaped the Saxons to set up a stronghold in the west. He chose to build a castle on the southern slopes of Yr Aran, above Beddgelert (Gwynedd). Construction began. However, every morning the previous day's work was found demolished. Vortigern's magicians told him to seek a boy with no father, born of the fairies. He would be able to solve the High-King's problem. Vortigern's men searched far and wide and discovered such a boy at what was soon to become Caer-Fyrddin (Carmarthen). His name was Myrddin Emrys, or "Merlin" for short. Merlin revealed that at night the mountain shook so that all buildings collapsed, because beneath it were buried two fighting dragons. One white representing the Saxons and one red representing the British, and the white one was winning! Afraid of such an omen, Vortigern fled.