The first time we find the character Merlin, is in Welsh poems. There he is called Myrddin. It is this Myrddin and a prophet boy Ambrosius Aurelius (who appears in the stories of a monk called Nenius) on who Geoffrey Monmouth bases Merlin. Geoffrey Monmouth changes Myrddin into Merlin, because he doesn't want the link between his character and the French word “merde”.

The story tells about king Vortigern who tries to build a castle on a hill. But every night the walls fall down and they need to start over every single morning. Vortigern is told by his fortune-teller to find a boy with no father. If he sacrifices the boy, the walls won't fall down any longer. He searches the whole land and finds a boy whose mother claims that she was pregnant out of nothing. Vortigern takes the boy (in Nenius stories, the boy is known as Ambrosius) to his castle. There he wants to slaughter the boy, but Merlin tells the king that there are dragons under the hills. A white and a red dragon are fighting, so he says . Vortigern digs out the hill and they find the two dragons. They witness the final battle between the two and see how the red dragon wins. Merlin told Vortigern that the red dragon stood for the Saxons and the white dragon for the Brittons1.

Merlin predicted the doom of Vortigern and claimed that Aurelius Ambrosius and Uther were the rightful princes. In the beginning Merlin is put into the story to predict Arthur's reign. There is no serious doubt that Geoffrey made up most of the prophecies of Merlin. Geoffrey puts in a note that he had collected some prophecies by Merlin about events in Britain2.

Geoffrey of Monmouth started to write History of the Kings of Britain , in which the story above is set. He speaks about Merlin in the beginning and is intending to deal with the sayings of Merlin later. However, stories about Merlin began to circulate from other sources, and he was urged by his friends and other to complete the book Prophecies of Merlin first3.

According to some people, Geoffrey misked up the stories of Myrddin (also known as Merlin Sylvster) and Merlin Ambrosius (the prophet boy in Nenius' stories).

In Malory's Morte d'Arthur Merlin's role has changed. Arthur is a bringer of order in the chaotic land, with help of Merlin's magical aid2.

Tennyson added Nimue as the wife of Merlin. He also said that Merlin had an affair with Vivien. In this story Merlin stands for the intellect, and his disastrous affair with Vivien is symbolic of the corruption of the intellectual by the sensual3.