The classic unseen demon is fascinating because it taps into a primal, psychological fear that's often more potent than any CGI monster. It's rarely about a list of attributes; it's about absence and inference. The audience, alongside the characters, has to imagine the threat from scattered clues—a sudden drop in temperature, the smell of ozone and rot, a voice that seems to come from the walls themselves. That act of co-creation with the story is what gets under your skin.
I find the most effective ones operate on rules that feel both ancient and arbitrary, which makes them feel genuinely alien and uncontrollable. They might be bound to a place, an object, or a bloodline, but their motivations are rarely human. They don't want to rule the world; they want to unravel it, or feed on despair in a way that feels ecological. Their presence warps reality subtly at first—displaced items, distorted reflections—before escalating to full-on psychological torment. The horror isn't in the jump scare, but in the slow, sinking realization that the entity was already there, listening, long before anyone noticed.
That's why stories like 'The Haunting of Hill House' or the 'Grimoire' mythos work so well. The demon isn't a character you meet; it's the atmosphere, the history of the house given a malignant will. It wins by making you doubt your own mind, which is a far more intimate violation than any physical attack.