High Priest

The High Priests are a group of sorcerers who serve the Old Religion and head all the other warlocks. They are the male equivalent of the High Priestesses, though it seems there are less High Priests than Priestesses. It's also possible that in the priestly hierarchy of the Old Religion the female servants are more important than the male ones, as, for example, when Gaius talked about the time of the Old Ways, he always mentioned the High Priestesses and not the Priests.

It's unknown how many High Priests are left after the Great Purge and if all of them are now enemies of Camelot. Alator declared that he had been hunted for many years by Uther's men, but he chose to help Merlin in his struggle to bring about Albion's time.

The Catha
The Catha are warriors and High Priests of the Old Religion. They have very powerful magic and possess secret skills known only by initiates, like the magical mental torture.

The Blood Guards, warriors and priests sworn to protect the High Priestesses of the Old Religion, may be part of the Catha.

Known Priests
The only confirmed High Priest and warrior of the Old Religion is Alator of the Catha. He lived in a arab-like city on an isle near the coast. There he had found refuge from Uther and his Great Purge. He was served by another man, likely a less powerful priest, Orn.

Although never mentioned or explained, Gaius may have been a priest of the Old Religion (but more likely their servant and allay). He was a friend of Nimueh, servant of the Triple Goddess, and knew the other High Priestesses: in fact, he gave them the infant Morgause after having smuggled her out of Camelot (The Sins of the Father).

Possibly, the ferryman who took Morgana and Morgause to the Isle of the Blessed was a priest too. He was seen using magic, making his boat move on the waters of the lake without having to paddle (The Darkest Hour).

Its also possible that the druid cheiftains are also (or a type) high priests, as both chieftains seen so far have been male, and they also share the great power and knowledge of the old religon. If so then Aglain and Iseldir were also high priests. (The Nightmare Begins, The Coming of Arthur)