The cult of Nar'sie is a cult of sacrifices. It doesn't quite matter what. Blood, bone, lives, locations. Nar'sie demands something be lost in exchange for something to be gained, but it doesn't have to be something the cultist owns, which means followers of Nar'sie often turn to murder, and as such are often known casually as the blood cult.
They borrow power from Nar'sie, a powerful extradimensional being, in return for gathering more influence for her, eventually resulting in summoning her into their realm to reap more followers. Many cultists are 'brainwashed', but just as many may be genuine followers.
Many blood cultists are driven on instinct, relying on the impulses Nar'sie communicates with, and allowing emotions to heighten their borrowed powers. Their deity can revive them nigh-indefinitely as long as she has enough influence to hold her attention, making them dangerous foes, especially as they work in groups. The final summoning attempt often takes place after a long period of raising the emotional state and instinctive fear of a target to make them rich in influence, and sacrificing them to begin the summoning.
Nar'sie herself thrives on chaos and sacrifice, and values acting freely (of course, as long as it benefits her.) Without the efforts of her cultists, her sphere of influence is limited to that murky water magic users reach deep into for power. Said magic users are most likely how she gained her foothold in the waking world in the first place, and spread her belief like a contagion.
[add more later, check notes]Due to their similar goals, the cult of Nar'sie is often at odds with the cult of Ratvar, and the two deities will fight whenever they encounter each other in the physical world.
Inspired by https://hackmd.io/@shiptest/HyagVQnPq