They are effective because they quickly create a mood. For example, 'The door opened by itself. A shadow slipped inside.' Just these two sentences can make you feel uneasy.
Here's another. 'I found a doll in the attic. Its eyes followed me as I left.' This creates a sense of unease in just two sentences. The idea of a doll having some sort of awareness and the fact that its eyes seem to move on their own is quite creepy. It plays on the common fear of inanimate objects coming to life or having an otherworldly presence. In many horror stories, dolls are often used as a symbol of something sinister because they can look so human - like yet be so lifeless, and this two - sentence story captures that essence of horror.
The first one: I woke up in the middle of the night. When I looked at the mirror, there was a figure behind me but no one was there when I turned around.
Story: 'I woke up to find my name written on the wall in blood. No one else was in the house.' Answer: The idea of finding your name written in blood is very creepy. And the fact that there's no one else in the house makes it even scarier as it seems like a supernatural or very dangerous entity did it.
One example is 'My sister said she saw mom in the basement. Mom died last year.' It's horrifying because seeing a dead relative in a place like the basement is really spooky.
The brevity of these stories is key. They cut straight to the horror. Like 'I found a doll in the attic. Its eyes followed me.' There's no time for build - up, just pure, sudden horror. It's like a quick jolt to the senses, and our minds are wired to be scared by the unknown that these two sentences hint at.
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.