What makes a good horror story?
You will say ghosts, monsters, vampires, etc. As a classic horror author H.P.Lovecraft. "The oldest and strongest kind of fear is fear of the unknown". Writers did not reveal the horrors but by leaving the audience hanging in anticipation i.e. in a state of suspense. The most familiar examples of tension come from horror films and mystery novels. The key to suspense is it sets a question in the hope audience will get the answer too by maintaining the interest and keeping them guessing. Next limit the Point of view, and tells the story from the perspective of the characters start as they know little as the audience does as they learn more. Just keep audience in dark but sometimes reveal key parts of the big secret to the audience. But not to the characters. This technique knows as Dramatic Irony. Classic play "OEDIPUS REX" the title character is unaware that he has killed his own father and married his mother but the audience knows.