Topic: What makes a game extremely scary?

Another thing on one mentioned is conditioning. I've got two good examples of this:
First, say a difficult/invincible enemy appears/jumpscares you whenever the music swells. After this happens a few times, you will find that music absolutely terrifying, whether or not anything jumps out at you, or even if you are just listening to the song without playing the game.

The second is probably a better example, but done the other way around. First, the game makes you accustomed to some sort of pattern over the course of the game. Then, when it wants put you on edge for a jumpscare or just disturbing moment, it throws a wrench in to the formula. It seems at first like what usually happens, but something just feels off. And that's exactly when the game throws everything at you it has.