Nimueh

Nimueh was a Priestess of the Old Religion, an opponent of Uther Pendragon, and the main antagonist of the first series. She devised a number of schemes to wreck his kingdom before she turned her attentions onto Merlin, a young warlock, and Arthur, Prince of Camelot.

Biography
Twenty years before Merlin arrived in Camelot, Nimueh was a High Priestess of the Old Religion. She and Gaius practiced magic together. She was friends also with King Uther Pendragon. Uther, desiring a son and heir, realised that Ygraine was incapable of providing one and asked Gaius to approach Nimueh with a request that she use the power over life and death to help his wife get pregnant. She did so, and as a result Arthur was born. However, according to her, a life had to be taken to restore the balance of the world in accordance with the laws of the Old Religion, and because of that Ygraine died in childbirth. Uther blamed magic in general and Nimueh in particular for her death and began the Great Purge, driving all who practised sorcery 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.

Personality
Nimueh appears as an embittered and selfish witch who wishes to bring ruin on Camelot and Uther Pendragon in revenge for the execution of numerous magic users, including her friends and family. 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. She seems to play a part in the Great Purge since she obviously was capable of choosing who had to die in exchange for another life, hence taking Ygraine's life and causing Uther's wrath. In Excalibur, Nimueh claimed that she did not know that Ygraine would die but that the Old Religion did not care who would live or die, yet it is unlikely that she did not know since she was a Priestess of the Old Religion and also a seer. She was also blamed by Gaius and Merlin to be the one who chooses who had to die to which she did not object but proved them right when choosing Gaius' life in Le Morte d'Arthur in exchange for the life of Merlin's mother Hunith. Even though Uther was warned that someone will have to die for the life of his son, Nimueh did not tell him that it would be someone close to him. It is unknown why Nimueh obviously chose Ygraine intentionally. However, the life for a life-policy seemed to be continued only as long as the Questing Beast appeared, a creature of the Old Religion that appeared on the day of Ygraine's death as well as when Arthur was mortally wounded by it and who was later rescued by Merlin when he killed Nimueh. After the Questing Beast and Nimueh's death, numerous lives were saved at the hands of magic, yet without the demand of taking another life in exchange.

Abilities
As a High Priestess of the Old Religion, Nimueh was a very powerful sorceress who mastered control of the elements as well as the power over life and death. She claimed to have the power of prediction, suggesting that she was a Seer. Using a scrying fount, she's able to remotely view events as they occur. She may also have been 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 in possession of the Cup of Life, and had access to its tremendous power, something many other sorcerers (like Morgause) longed for.

She was able to easily stop Merlin's magical attack on her during their final confrontation by absorbing it into her hand. However, she didn't manage to kill him because of his natural resilience to magic and he 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.

She was also able to use the elements of water and earth in order to create an Afanc. Moreover, it is assumed that she had a certain knowledge of magical plants as she used an enchanted Mortaeus flower to poison Merlin. She had the power of necromancy, which she used to created a wraith using Tristan de Bois' corpse. She used the element of fire to create deadly fireballs. Finally, she was able to control at a very high level the power of life and death due to the fact that she tricked Merlin by taking his mother's life instead of his.