Effective extremely scary two - sentence horror stories often use the fear of the dark. 'I turned off the lights. In the pitch black, I felt something breathing on my neck.' Darkness is something that humans are instinctively afraid of. Also, the use of the abnormal. 'The mirror showed my reflection. But then, my reflection blinked when I didn't.' This abnormal behavior makes it scary. And the feeling of being watched. 'I sat in the empty park. I couldn't shake the feeling that someone was watching me from the bushes.'
Two-sentence horror stories can be quite scary. They often use simplicity and ambiguity to let your imagination run wild, which can make the fear more intense.
There's a horror story where a girl received a strange old doll. Every night, she would wake up to find the doll in a different position. One day, she noticed the doll's eyes seemed to follow her. As time passed, she started having nightmares of the doll coming to life and strangling her. She finally threw the doll into a river, but that night, she heard the doll's laughter outside her window.
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.