A two - line horror story is effective when it creates immediate fear. For example, 'The phone rang at midnight. It was my own voice on the other end.' This short scenario plays on our fear of the unknown and the uncanny. It gives just enough information to spark our imagination and make us feel uneasy.
Walking through the cemetery, I heard a soft cry. I turned to see a little girl in a white dress. When she looked up, her eyes were black holes that seemed to suck the life out of me.
The element of surprise. For example, 'I sat on the park bench. The empty bench beside me whispered my name.' Just a simple, unexpected twist in the second line can be very effective.