structure of this religion is yet to be explored, but it is likely that the High Priestesses are a group of sorceresses who held control over the other wielders of magic, and were worshipped by them.
The High Priestesses were the main servants of the Triple Goddess. They were a group of skilful and powerful sorceresses, mistresses of magic and spells and detained the power over life and death. They probably lived on the Isle of the Blessed, where they guarded the Cup of Life (at least, it is clear that the Cup was in Nimueh’s possession during Series 1. It is unknown, though, how the Druids got their hands on it after her death). They were served by the Blood Guard and they guarded the Rowan Staff, a powerful magical instrument carved from the Rowan Tree that grows at the very heart of the Isle of the blessed. The Rowan Tree was very important to them and is the symbol of the Blood Guard.
On Samhain Eve, at the time of the Old Religion, the High Priestesses performed a blood sacrifice to open the Veil between this world and the Spirit world and release the Dorocha. They also used dark magic to summon form the depths of the Underworld the Fomorroh, a creature that they revered. This would allow them to take over the minds of their enemies, binding them to their will (A Servant of Two Masters).
It is possible that the Crystal of Neahtid was kept and guarded by the High Priestesses on the Isle of the Blessed too, before it was taken by Uther and locked into the vaults of Camelot. The many sorcerers who died trying to protect it could have been the High Priestesses and their followers (The Witch's Quickening).
According to Gaius, a long time ago, a war broke out between the High Priestesses and the ancient Kings. The sorceresses took the blood from a girl and mingled it with that of a serpent. The creatures they created had ferocious powers. They could control the mind of a man, suck the life from him and transform at will in hideous monsters. Eventually, the High Priestesses lost control over the Lamia, who went on killing everyone (Lamia).
When Uther and Camelot turned against magic and the Old Religion with the Great Purge, most of the High Priestesses were wiped out and slaughtered. The ones that survived hid themselves away, and from that moment they used their powers to try to bring about Camelot’s downfall.
Until the episode The Secret Sharer, it was unknown how many High Priestesses were left after the Great Purge. Anyway, Alator of the Catha, warrior and High Priest, revealed that Morgana was the last remaining High Priestess of the Triple Goddess. The other High Priestesses before Morgana were:
It remained unclear whether or not Morgana was a High Priestess during Series 3, though it seemed unlikely as her control over her powers was in its infancy. However, being Morgause’s sister, the sorceress’ Blood Guards (at the entrance of Morgause's cave) treated her with reverence.
It became clear that during her time away from Camelot with a weakening Morgause, Morgana had become a High Priestess, having mastered many different aspects of magic and proving to able to perform the blood sacrifice of Samhain (The Darkest Hour). Morgana gained full control of her magic, probably becoming as powerful as Merlin, but less experienced. A fight between the two was closely matched, though ultimately Merlin, even in his aged form, was able to defeat her in a magical duel (A Servant of Two Masters).
Morgause, Vivienne’s daughter, was smuggled out of Camelot and given to the High Priestesses by Gaius, who was probably their friend and maybe their servant, when Uther ordered her execution. The remaining High Priestesses taught her magic and how to master her powers. They probably instilled in her the hatred for Uther, Camelot and Arthur, and told her about her half-sister Morgana and her parentage. In the beginning of the episode The Sins of the Father, Morgause was found living in a ruined castle. It is possible that there is where she was brought and where the High Priestesses dwelt. Morgause was an extremely powerful sorceress, skilled in many different aspects of magic, with a particular brutal force. She was able to wield the Cup of Life and mirror life and death.
After her magical duel with Merlin and Gaius, she was badly wounded and condemned to a slow and painful death (The Coming of Arthur). Not wanting to live like a cripple for the rest of her short life, she decided to let Morgana sacrifice her on the Isle of the Blessed, so that the Witch could tear the Veil between the worlds. She had spent a year teaching Morgana magic, and after her death her sister became High Priestess (The Darkest Hour).
NimuehNimueh was an hydromancer and probably the last remaining High Priestess before Morgause. She had been a friend of Uther and Camelot before the Great Purge and the court sorceress. Her bargain with the King and the following death of the Queen started Uther’s war against magic. She tried many times to destroy the kingdom, but she was killed by Merlin with a lighting bolt on the Isle of the Blessed. Nimueh was an extremely gifted and powerful Priestess. She dwelt on the Isle of the Blessed and guarded the Cup of Life. With this magical vessel, she could summon the gods of the Old Religion and alter the balance of the world, harnessing the power over life and death to her own (Le Morte d'Arthur). She was also the only High Priestess to discover Emrys’ magic.
- It is possible that Vivienne, Morgana and Morgause’s mother, had been a High Priestess. In the episode The Secret Sharer, Morgana told Alator that her healing bracelet had been forged on the Isle of the Blessed by a High Priestess. This bracelet had been given to Morgana by her sister, who had told her it belonged to their mother. Considering this, the owner and the maker of the bracelet may have been the same person (Vivienne).
- Interestingly, Nimueh is the only High Priestess whose eyes have not been seen glowing gold when performing magic. She may have been trained so well that such a concentration was not required, or she just coincidentally was never seen performing spells that made the eyes glow (another example of magic that did not require this was Morgause’s column of fire in The Castle of Fyrien or while the High Priestess was using the Cup of Life in The Coming of Arthur) but it it is also possible that the proudercers only made a mistake with that because that has happened more than once.
- So far all three High Priestess have worn red at one time or another.