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 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. They were also trained from birth, to endure all suffering and pain both physically and mentally.
The Blood Guard, an order of warrior priests sworn to protect the High Priests and Priestesses of the Old Religion, may be part of the Catha.
- Alator †
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 (or their servant and ally). 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). Anyway Gaius declared that, long before the Great Purge, he had taken part in the rituals held on Beltane at the Great Stones of Nemeton, in which the High Priestesses used The Horn of Cathbhadh to summon the spirits of their ancestors, thus implying that he had indeed served directly the Mysteries (The Death Song of Uther Pendragon).
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).
It's also possible that the Druid chieftains are also (or a type of) High Priests, as both chieftains seen so far have been male, and they share the great power and knowledge of the Old Religion. If so then Aglain and Iseldir were both High Priests (The Nightmare Begins, The Coming of Arthur).