The element of surprise. For example, in 'I opened the door. A monster was waiting on the other side.' It hits you suddenly. There's no long build - up, just immediate horror. It forces your mind to fill in the blanks and imagine the worst - case scenario, which is often scarier than if it was all spelled out.
Here is one. 'The old house was silent. Then I heard the sound of a child laughing from an empty room.' This simple two - sentence horror story creates a spooky atmosphere. Just imagine being in a quiet, old house and suddenly hearing a child's laughter from a room that's supposed to be empty. It gives you a sense of unease and mystery.
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.
They are concise. For example, 'The phone rang in the dead of night. It was my voice on the other end asking for help.' In just two sentences, it creates a sense of dread. They often rely on the unexpected. Like 'I found a door in my basement that wasn't there before. When I opened it, a cold hand grabbed mine.' It takes a normal situation and twists it into something terrifying.
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.
Some two - sentence horror stories are truly spine - chilling. For example, 'I woke up to a noise in the kitchen. When I got there, all the knives were floating.' The simplicity of the setting and the unexpected, supernatural element make it effective. Another one could be 'She heard a baby crying in the attic. She doesn't have a baby.' It plays on our primal fears of the unknown and the unexpected in a very short space, creating a sense of unease and horror.