Demon Worshipers are people who worship the devil and his demons, similar to Satanists and other demonic cults. They create places marked by pentagrams and routes covered with ancient writing and artifacts that are all over their ritual domain.
They appeared as ghosts in the episode, "The Long Island Terror," where they have turned the house, where a single mother and her two daughters moved into, and the surrounding forests into their unholy site. There were written books that describe their horrifying rituals. After they have disappeared from the home, they had summon a demon that resided in the house who only allowed other worshipers to live there.