A Detective and his partner, have been given a murder case to investigate, A series of ritualistic, and brutal murders. The suspect is believed to be either a crazed psychopath, or a Satanic cult. As they go deeper in the investigations, they meet someone at a recent crime scene, who claims to be a "Demon Hunter", and that he has tracked this Cult for months, to stop them from summoning a powerful demon. They soon find out he wasn't joking.
Okay, about plot points, city/town and characters.
I essentially have an idea for one of the Detectives, and possibly the antagonist, Leader of the Cult and such. The Detective, well one of them would be definitely older, and more experienced, often seeing cases sort of similar to the one they are investigating. The imagery, and how brutal the murders leave a bad taste in his mouth, and thus he is the one to believe half of the Demon Hunter's story, in which he thinks they are delusional and such. I'm using "he" but obviously it can be a "she" too. The Villain? well he's pretty much a charismatic man, "his words are his weapons" probably best explains how he is to others. He's done this ritual before, albeit unsuccessfully because of outside interference.
Essentially the case does not become more hectic until they first meet the Demon Hunter, this will lead them to think that he/she is a prime suspect in this murder case.
I'm not sure if we should do a fictional city or an existing one?