Nimueh

Nimueh is a powerful and ancient sorceress who is one of Merlin's chief antagonists.

Biography
Twenty years before the series is set, she was a high priestess of the Old Religion. She and Gaius practised magic together and she was a friend of King Uther Pendragon and his wife Igraine. When Uther desired a son, having realised Igraine was incapable of providing one, he asked Gaius to approach Nimueh to use her magic to help Igraine get pregnant, which she did. The result was Arthur. However in bowing to the laws of the old religion a life had to be take to give a life - as a result Igraine died. Uther blamed magic for her death and began The Great Purge on all those who practised magic.

While Gaius conformed to the new religion, Nimueh was banished from the court, and vowed revenge for the death of all her friends. (Excalibur, Le Morte d'Arthur)

When she first appears she creates an Afanc, a creature of clay (earth) and water to plague Camelot by poisoning the water supply. Her plan is thwarted by Merlin when he destroys the Afanc using fire and wind. (The Mark of Nimueh)

Nimueh swears revenge on Merlin and poisons a chalice she knows he will be forced to drink by Uther. The chalice, supposedly intended for Arthur, is played on Bayard - a king who had come to Camelot to sign a peace treaty with Uther. She also leaves Arthur to die in the caves of the Forest of Balor but he is saved by a light sent by Merlin. (The Poisoned Chalice)

She attempts to kill Uther also by bringing back to life his brother-in-law Tristan De Bois, who swore to return from the dead and avenge his sister's death, as a wraith. However when Uther uses the sword Excalibur (Forged by Merlin and the dragon for Arthur) he defeats Tristan. (Excalibur)

Arthur is bitten by the Questing Beast, Merlin travels to the Isle of the Blessed and offers his life in return for Arthur's, although Nimueh warns him it will not be that simple. Arthur is brought back by the waters from The Cup of Life. However instead of Merlin, it is his mother Hunith who is sacrificed. Gaius tries to sacrifice himself for Hunith so Merlin cannot.

When Merlin arrives Gaius is dead. Nimueh tries to convince him to join with her but Merlin refuses. The sorcerers fight and Merlin is hit by Nimueh's magic, seemingly killing him. However Merlin survives, stands up and summons the power of the old magic, striking Nimueh dead thus saving Gaius. (Le Morte d'Arthur)

Personality
Nimueh appears as an embittered and selfish sorceress who wishes to bring ruin on Camelot and Uther Pendragon in revenge for the deaths of her friends. She appears to have some level of vainity as she maintained her youth most likely through use of magic.

Abilities
As a high priestess of the old religion Nimueh is a very powerful sorceress who has mastered control of the elements as well as the power of life and death. She claims to have the power of prediction, suggesting that she is a seer. She was able to stop Merlin's attack on her during their final confrontation. However she was not powerful enough to kill Merlin, most likely because of his own level of power, or prevent him from ultimately killing her. Furthermore, despite her power of prediction, she is at first unaware of Merlin's "greatness" (Also alluded to by the dragon).

Behind the scenes
Nimueh is based on the legends of the Lady of the Lake. In many versions of the legend she is responsible for the death of Merlin by trapping in a tree or cave. As the Lady of the Lake she is also credited with giving Arthur his sword Excalibur as well as taking it back after his death.

There is little similarity between the Nimueh of the legends and the Nimueh Merlin as not only did Merlin apparently killed her (Rather than vice versa) but Excalibur was created by Merlin taking a sword Guinevere's father made to The Great Dragon and forged with his breath.