In Dark Souls, Priscilla is a dragon-human cross-breed who resides in the painted world of Ariamis. As an abomination she had no place in the real world, and after clutching her Peculiar Doll, she was drawn into a painting. She has a tail, is much taller than a normal human being, and wields a scythe. When the player speaks to her she asks them to leave Ariamis. At that point one can either do as she wishes or attack her, at which point a boss fight with her will commence.
Priscilla's dagger is obtained by cutting her tail off. Priscilla's soul can be used to craft the Lifehunt Scythe. It can be inferred through various NPC dialogues and item descriptions that Priscilla was banished to the Painted World due to the gods' fear of her Lifehunt ability.