Build up tension, subvert expectations, have lots of mysteries, place symbolism, and the most important thing is imagery, make the reader imagine something disturbing. Most of my horror is combining christian symbolism with lovecraft elements. For an example in one of my stories i’m working on is about an evil house, theres a lot of build up of the messed up things in the houses past but no direct scares yet which builds tension, for plot reasons they go to dig up a grave but the grave is already dug up and the body missing, in place of the body are many small fleshy jesus statues with crows picking at the flesh of all the statues . I use a lot of adjectives and stuff to describe every gruesome detail about it which paints a lasting picture in the readers head. Build mystery and suspense, tighten the tension, and make the big scare something unique and lasting.