The Warpriests are a mercenary order founded in the late 600's centering around fighting for Zelial in numerous wars.
They were founded by Hetman-Bishop Jacques in an area of northern Ravenna, where Jacques was a bishop for a local church dedicated to Zelial. Jacques, who was known to smoke a ton of opium came up with the concept one day and created the organization in the span of a week. It grew pretty well for a group made by a bishop who was cracked out of their mind, recruiting churchgoers and local thugs ready to beat people up for a holy cause.
The rule of Jacques didn't last long, as the group soon become known to The Cult of Zelial, who silently killed Jacques and replaced him Elizabeth, the newly ordained Hetman-Bishop who was an operative of the cult. The Warpriests weren't immediately subsumed into the larger cult structure because Elizabeth persuaded the higher ups in the cult that there was a sign of promise in them, and that it would be smart to keep an external organ of a puppet group in case they needed to invisibly tip the scales of a conflict.
So, under the rule of the Cult of Zelial, the mercenary groups went international and grew to be a major mercenary group in the world, with recruits spanning from The Eari Isles to the Republic of Valdar. Their most major contribution was during the Blood War. While most mercenary groups were on the side of the Uthani, the Warpriests deferred to the will of the cult, who were most certainly hostile toward the Uthani, and thus fought against them. Their most significant contribution was at the Battle of Catyiu Heights, defeating an Uthani army who outnumbered them three to one.
Most mercenary groups are either composed of fighters of a physical kind or mages. The Warpriests took the middle path and chose both, meaning their warriors not only know how to beat someone up with a mace or a sword, but also know how to fire off fireballs at approaching enemies or otherwise strengthen themselves using magic. They have little to no formal ranged support like archers or crossbowmen, relying on their own magic to do the trick at harassing enemies at long ranges.
At the top of the hierarchy is the Hetman-Bishop, who is always a member of the Cult of Zelial, who is sent in to keep an eye on the mercenary group and make sure they're doing exactly what the cult wants them to do. Other than that, they don't really do much else other than giving orders and sitting back. Second in the chain of command are the Commanders, of which there are four. Their job is to both command the Warpriests in a grand strategy view and to do all the administrative work that the Hetman-Bishop doesn't do. Next are the Colonels, who command the soldiers in the field directly, often being on the front lines along with their men.