The scariest elements often include the unknown. For example, in stories where there is something sensed but not seen clearly. Like 'I felt a presence in the dark corner. When I turned on the light, the presence was gone but a cold draft remained.' Here, the unknown presence is very frightening.
Sudden and unexplained changes are also scary. Such as 'The photograph on the wall was normal. Seconds later, the faces in it were twisted in agony.' It's the quick and unexplainable shift that makes it terrifying. These elements work well in two - sentence horror stories as they can quickly build tension and fear.
Isolation is another key scary element. Consider 'I was alone in the old house. I heard a voice whisper my name from the empty hallway.' The feeling of being alone and something menacing happening creates a sense of dread. In these short horror stories, isolation heightens the fear factor as there is no one to turn to for help.
The brevity. They hit you with the horror quickly without giving you time to prepare. For example, 'The phone rang at 3 am. It was my dead sister's voice.' Just two sentences and it's already very scary.
Another really scary one could be 'My phone rang at 3 am. The caller ID said it was me.' It plays on the idea of the self being something other or unknown, and calls at that time are always spooky.
I think 'The clock stopped at midnight. Then I heard footsteps.' is quite scary. It gives this feeling of something ominous starting right at a spooky time.
I think 'I heard a child crying in the basement. But I live alone.' is quite terrifying. The idea of a crying child in a place where no child should be, especially when you live alone, plays on our fears of the unknown and the supernatural. It makes you wonder what could be down there, and the thought that it's not a normal, explainable situation is very frightening.
The sense of helplessness is a major scary element. Like when a character realizes that there's no way to escape from El Coco once it has arrived. It gives a feeling of doom and despair. For instance, in a story where a person is trapped in a small space and El Coco is approaching, they know they can't get away. This powerlessness is what makes these stories truly frightening.
The sudden appearance of unknown entities. For example, a figure suddenly showing up in a supposedly empty room.
The sudden and unexpected events. For example, a character thinking they are alone and then suddenly seeing a shadowy figure right behind them.
I walked into the empty room. Suddenly, I felt a cold hand on my shoulder, but no one was there.
There was a knock on the door. Nobody was there when I opened it.
I woke up to find a shadow at the end of my bed. When I turned on the light, it was gone.