What is the most terrifying "Rules-Based" paranormal entity in a movie?
I’m looking for movies where the haunting or entity operates on a strict, bizarre set of rules. I don't mean just a standard poltergeist throwing plates. I'm talking about stuff like It Follows (the entity walks, you have to pass it on), the tape in The Ring, or the specific demonic attachment rules in The Conjuring universe.
I love the dread that comes from characters having to figure out the "mechanics" of the haunting to survive, only to realize they messed up one small detail.
What are your top movie recommendations that do this trope perfectly? I need something new to watch tonight.