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 control the other followers, 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 is a High Priestess or not. Maybe during Series 3 she wasn't, not being very skilled in magic, but by Series 4 she'll certainly be, being as powerful as Merlin and probably more powerful than Morgause.
 * Nimueh (deceased)
 * Morgause
 * Morgana
 * Vivienne (possibly)

Morgana and Morgause's mother, Vivienne, may have been a High Priestess too, even though there's no proof about it. It's known anyway 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 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.

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