Here are two. 'I found a photo of myself sleeping. I don't remember anyone taking it.' And 'I was home alone. The phone rang, and when I answered, all I heard was my own voice whispering things I don't remember saying.'
Well, it's the sudden shock. In just two sentences, they create a disturbing image. For example, 'I looked in the mirror. My reflection had a slit throat.' It gives no time to prepare, just hits you with the horror right away.
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 play on our basic fears. Things like the unknown, as in 'The attic door creaked open. A cold wind blew out, carrying a strange smell.' It makes you wonder what could be in the attic, and that uncertainty is what makes it scary. Also, they often use common horror elements in a very compact way, like the idea of something not being as it seems in 'My doll's eyes followed me around the room. I was the only one home.'
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.