Well, when reacting to two - sentence horror stories, the first thing that comes to my mind is the shock factor. These stories are so short yet they can send chills down your spine. For example, 'I woke up in the middle of the night. There was someone standing at the foot of my bed.' Just this simple description makes you imagine all sorts of creepy scenarios.
One story could be 'I always sleep with a nightlight. Last night, I woke up to find it off and a cold hand on my wrist.' Another might be 'The old doll in the attic blinked. Now it sits at the foot of my bed.'
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.
Another one could be 'I heard a scratching at the window all night. In the morning, there were deep gouges on the glass and no sign of what made them. The simplicity of two sentences can create a very intense and spooky atmosphere.
Here's another. 'I woke up to find the world silent. When I looked out the window, all I saw were shoes.' This is scary as the silence implies something is very wrong, and the sight of just shoes is strange and makes you wonder what happened to the people.
Well, 'My phone took a picture of me while I slept. In the photo, there was a figure standing over me.' This is quite scary as it involves the invasion of our private space, especially when we are at our most vulnerable while sleeping. It also adds the mystery of an unknown figure. This makes us wonder what that figure could be and how it got there while we were unaware.