Volumes could be written about the druid named Merlin but some say he is best known as the wizard featured in Arthurian legend and was most popular in Wales. He was a Cambion: born of a mortal woman named Adhan. He was sired by an incubus, the non-human from whom he inherits his supernatural powers and abilities, some of which included sorcery and shapeshifting.
He has appeared 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. He also 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. Finally as a wizard, he becomes an elder advisor to Arthur
Merlin's eventual downfall came from his lusting after a huntress named Niviane, who was the daughter of the king of Northumberland. She was about to depart from Arthur's court, but, with some encouragement from Merlin, Arthur asks her to stay in his castle with the queen. During her stay, Merlin falls in love with her and desires her. Niviane, frightened that Merlin might take advantage of her with his spells, swears that she will never love him unless he swears to teach her all of his magic. Merlin consents, unaware that throughout the course of her lessons, Niviane will use Merlin's own powers against him, forcing him to do her bidding.
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.
This is unfortunate for Arthur, who has lost his greatest counselor.