A great two - sentence horror story is 'I heard my name whispered in the empty hallway. But I was alone in the house.' Just the act of hearing your name when you're supposed to be by yourself is really creepy. It gives the feeling that there's an unseen presence that knows you, and it could be anywhere in the seemingly empty house, waiting to do who - knows - what.
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.
To write a spine - chilling two - sentence horror story, you need to play on people's fears. Start with something that seems normal but has a strange twist. Such as 'The cat came back home, but it had someone else's eyes.' In the second sentence, you can leave the reader with a feeling of doom. For instance, 'I realized then that whatever had taken over the cat was now in the house with me.'
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.
The lack of explanation. In 'two sentrnce horror stories', often things are left unsaid. Like 'My phone showed a call from my dead mother. It rang again.' It leaves your mind to fill in the blanks and that's really scary. Your imagination can come up with all sorts of terrifying scenarios.