Here are the basics
Important Basics:
Unique mechanics (Finding keys and mazes are not unique, just had to clarify that)
Atmosphere (A strong atmosphere makes people scared 99.9% of the time without using jumpscares)
Jumpscares/Scariness (Games with good atmosphere most of the time wont need these but if your making a long game with about 20m-1h play time then your gonna need these, Best form of them is usually from being caught by a enemy/Attacked. Other jumpscares such as touching an invisible brick and a image flashes on screen, To make jumpscares startle the player is to make the sound of the jumpscare fast and loud or make the jumpscare very fast, Not meaning go by fast even though that would work well for the touch brick jumpscare, Im talking about the the speed the creature/enemy is moving in the jumpscare, A slow jumpscare with slow sound probably wont scare the player as much as a fast moving creature shaking across the screen with a loud high pitch sound playing.)
Hopefully someone looking into making a horror game finds these basic tips useful.