High Priest

"[...] The High Priests have the power to mirror the life and death, but there will be a price to pay. They will demand a life in return."

- Gaius about the High Priests

High Priests are a group of sorcerers who serve the Old Religion. They are head of all other warlocks and are the most skilled and advanced in the hierarchy of warlocks. They are the male equivalent of the High Priestesses, who held control over all other wielders of magic, and were worshiped by them, though it seems that, after the Great Purge, there are less High Priests than Priestesses left in the world.

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.

Known High Priests

 * Alator †

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.

Druid chieftains (possibly)
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).