Nimueh

Nimueh was a sorceress who was determined to bring down Uther Pendragon and became a powerful enemy of Merlin before she was killed by the young warlock.

Early life
Twenty years before Merlin arrived in Camelot, Nimueh 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 that Igraine was incapable of providing one, he asked Gaius to approach Nimueh and request that she use her magic to help Igraine get pregnant; she did, and the result was Arthur. However, in accordance with the laws of the Old Religion, a life had to be taken to give a life, and Igraine died in childbirth. Uther blamed magic in general and Nimueh in particular for her death and began the Great Purge, driving all who practiced magic out of Camelot. 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)

Nimueh's Plague
She later exacted some measure of this revenge after Merlin began his apprenticeship with Gaius when she created an Afanc, a creature of earth and water, with which she poisoned the water supply of Camelot. Her plan was thwarted by Merlin, Arthur, and Morgana who destroyed the Afanc using fire and wind. Uther was dismayed when Gaius discovered a mark on the Afanc's egg that proved that the plague was Nimueh's handiwork. (The Mark of Nimueh)

The Poisoned Chalice
Nimueh swore revenge on Merlin for his part in foiling her plan, posing as the handmaiden Cara within the court of Lord Bayard of Mercia. When Bayard traveled to Camelot to sign a peace treaty with Uther, Nimueh disguised herself as a serving girl named Cara among Bayard's retinue and traded out a chalice originally meant for Arthur with one that she had poisoned, and inveigled for Merlin to drink from it. She blamed the poisoning of the chalice on Bayard in order to sow discord and incite war, but the truth was discovered by Gaius before Camelot and Mercia could resume hostilities. She later led an unwitting Arthur to the caves under the Forest of Balor when he ventured there to find the Mortaeus flower; she left him for dead as enormous spiders swarmed, but he is saved by a light sent by Merlin. (The Poisoned Chalice)

The Wraith
She later attempted to kill Uther by reanimating his brother-in-law Tristan De Bois, who had sworn to return from the dead and avenge his sister's death, as a wraith. Ultimately, Uther wielded the sword Excalibur – which had been forged by Guinevere's father, taken by Merlin to The Great Dragon, and imbued with ancient magic for use only by Arthur – to defeat Tristan. (Excalibur).

Death
When Arthur was bitten by the Questing Beast, Merlin traveled to the Isle of the Blessed and offered his life in exchange for Arthur's. Nimueh warned that such a trade would not be so simple; Arthur was brought back by the waters from The Cup of Life, but rather than sacrificing Merlin, his mother Hunith was taken instead, though Gaius attempted to sacrifice himself for Hunith to prevent Merlin from doing so. When Merlin returned, he found Gaius dead; Nimueh tried to convince him to join her, but he refused. The two battled and Merlin was hit by Nimueh's magic, which appeared to kill him. He survived, however, and summoned the power of the old magic to strike Nimueh with a lightning bolt, presumably killing her, and saving Gaius. (Le Morte d'Arthur)

Legacy
Arthur later learned from the spirit of his deceased mother that Nimueh had used magic to create him, but he was led to believe that this was a lie. (The Sins of the Father) Merlin also mentioned Nimueh when he and Gaius discussed the Cup of Life, saying that he thought the cup was destroyed when he killed Nimueh. Instead the Cup fell into the possession of a Druid tribe. (The Coming of 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 displays a mischievous exterior, and appears to have some level of vanity as well. Her youthful appearance is a mystery, though in The Poisoned Chalice, it is speculated that she can cast illusions through the use of magic.

However Nimueh was described as beautiful and loyal to Uther before the great purge.

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. Using a stone scrying fount, she is able to remotely view events as they occur. She may also be capable of magically communicating with individuals at a great distance, as suggested by her conversation with Uther the night before his duel with the wraith of Tristan de Bois.

She was able to easily stop Merlin's magical attack on her during their final confrontation, absorbing it into her hand. However, she didn't manage to kill Merlin because of his natural resilience to magical attacks and Merlin ended up seemingly slaying the powerful sorceress. Furthermore, despite her power of prediction, she was, according to the Great Dragon, at first unaware of Merlin's greatness.

Appearances
Series 1:

The Mark of Nimueh (first appearance)

The Poisoned Chalice

Excalibur

Le Morte d'Arthur(Dies)

Series 2:

The Sins of the Father (mentioned only)

Series 3:

The Coming of Arthur: Part One (mentioned only)

Behind the scenes
Nimueh is based on the Lady of the Lake, alternately known as Nimue(h), Viviane, Niniane or Nyneve. Her role in the legend includes giving King Arthur his sword, Excalibur, raising Lancelot and being Merlin's apprentice and lover. After Merlin taught her all his secrets, she used her power to trap him in the Crystal Cave, effectively killing him. Thereafter she was an ally of King Arthur's.

There is little similarity between the Nimueh of the legends and the Nimueh in Merlin, as not only did Merlin apparently kill 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. To further support this, in series three, it was Freya that gave Merlin Excalibur, and Freya had also seemed to become the Lady of the Lake after her death.