Brutal! An Undead Priest... What's the lore behind it?
The Cultists have mages located on Tier 3 and do not have a Mage Guild – this makes the wizards unreliable; they are ready to betray their own for knowledge.
The Cultists have had several major religious campaigns, one of which was aimed at the lands of the ancient humans – into the undead swamps.
The Cultists fortified themselves, and the Archons decided that a direct attack was too risky. The ancient undead began to seek out traitors and enlisted the help of mages.
The result: all the great Knight-Cultists fell under a curse, turned into vampires, and began to go crazy. The mages received immortality and freedom – now they are priests of the god of death. A plague swept through the land of the Cultists; those who accepted "help" from the priests became intelligent zombies, and those who rejected it became skeletons.
The Archons gained an allied sub-faction, but some mages realize that they have been deceived – they have immortality, but not much strength has been added + the vampire knights are not eager to obey, and most importantly, they can put up a decent fight. In general, the faction is divided and unites only in the face of a common enemy.
It should also be said that these events happened a long time ago, so the Cultists have already managed to fight each other for the throne and squandered all their armor-making skills – a warlord who relied not on people, but on rituals, won. The Cultists expanded their army with spirits and mutants, but lost their armored knights and archers.
I plan to replace the mage-cultists with Nagas, as mutants are less prone to betrayal