Whether simple religious zealots, warlocks who've made dark pacts with entities outside our realm of comprehension, or powerful demons who've been summoned to lead the forces of darkness, cultists are all devoted to their cause.
Any inteligent being can fall to the temptation of darkness, and become a Cultist, but most are humanoids, especially humans, for whom religious zealotry is nearly second nature. Boasting a wide variety of believers and abilities thereof, they are often driven underground by their dark practices arousing the ire of kingdoms and populace.
Sacrifices, summoning demons, the creation or finding of evil artifacts; these are all things common to cultists. The members of the cult often need to reaffirm their belief, and many join for the activities of the cult first, rather than true belief. As time goes on, though, their faith grows stronger and stronger.
Common humans chosen for their belief, rather than ability, make up the majority of Cultist ranks, but those skilled in magic can advance in rank from simple adept up through warlock, and eventually, sorceror; archmages are possible, but rare and incredibly powerful, often having used their powers to enhance or completely chance their physical bodies. The priesthood is separate, acolytes graduating to priests who eventually become high priests. Every priest aspires to eventually be made a dark avatar of their god, filled with power, at the mere price of their soul, mind, and spirit, emptied to make room for the divine power that consumes them. The physically adept can become recruits, then warriors. From here, they typically follow one of several routes of advancement; cult guard, who are a sort of honor guard and protective group for religious ceremonies and the clergy, who're overshadowed somewhat by their superiors, called blackguards, who can eventually become death knights. Or, warriors might be promoted to the blood guard, instead, who function as the more offensive rather than defensive arm of the Cult. The senior members of the blood guard are referred to as blood knights, who can eventually become the ravaging berserkers. Warriors skilled in archery can become dark hunters, who graduate to dark rangers. The most skilled dark rangers can become the deadly snipers. Finally, those warriors more suited to the cloak and dagger become shadow stalkers, who become assassins, the best of whom are known as night killers. And, of course, the cult has it's demonic allies to call on, once the proper buildings are set up, sacrifices made, and magic rituals enacted.