High Priestess

The High Priestesses are probably the most important figures of the Old Religion. The structure of this religion is yet to be explored, but it's likely that the High Priestesses are a group of sorceresses who held control over the other wielders of magic, and are worshipped by them.

It's unknown how many High Priestesses are left after the Great Purge. The known High Priestesses are: It's actually not clear whether Morgana was a High Priestess or not during Series 3. Probably she wasn't, for she had only learnt how to control her powers but not how to fully use them. By the events in Series 4 she certainly is, having mastered, under her sister's teachings, many different aspects of magic and sorcery, and being able to perform the blood-sacrifice of Samhain.
 * Nimueh (deceased)
 * Morgause (probably deceased)
 * Morgana
 * Vivienne (possibly)
 * Medhir (possibly)

Morgana and Morgause's mother, Vivienne, couold have been a High Priestess as well, even though there's no proof about it. It is, however, known that she was a witch and that her daughters inherited their magical powers from her.

Morgause was smuggled out of Camelot, and given to the High Priestesses when Uther ordered her execution by Gaius, who was probably their friend and maybe their servant. In the beginning of the episode The Sins of the Father, Morgause is found living in a ruined castle. It's possible that there's where she was brought and where the High Priestesses dwelt.

The High Priestesses are a skillful sorceresses, mistresses of magic and spells, and they detain the power over life and death. They probably live on the Isle of the blessed, where they guard the Cup of Life (at least, it's clear that the Cup was in Nimueh's possession during Series 1. It's unknown, though, how the Druids got their hands on it after Nimueh's death). They are served by the Blood Guards 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 is very important to them and it's the symbol of the Blood Guards.

On Samhain Eve, at the time of the Old Religion, the High Priestesses performed a blood-sacrifice to open the veil between the worlds and release the Dorocha.

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 known High Priestesses still alive have turned against Uther and Camelot after the Great Purge.