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.'
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.
The brevity. With just two sentences, they create a sense of immediate horror. There's no long build - up, just a quick punch of fear. For example, 'I woke up to find a handprint on my mirror. It wasn't mine.' It makes your mind race with questions and fears right away.
One could be 'I woke up to find my reflection smiling back at me, but I wasn't.' It creates an immediate sense of unease as it plays on the idea of something being wrong with one's own reflection, which is often associated with the self. It makes you wonder what has taken over your appearance.
To create a terrifying two - sentence horror story, it's important to set a mood quickly. You can do this with your choice of words. For instance, 'The cemetery was quiet. Then, a bony hand reached out from the ground.' The first sentence gives a setting, and the second sentence disrupts that normalcy with something terrifying.