Use vivid and disturbing imagery. For example, instead of just saying 'There was something in the corner', say 'In the corner, a hunched figure with long, gnarled fingers twitched and stared at me with empty eye sockets'.
Build suspense between the two sentences. In the first sentence, set an ordinary or slightly uneasy scene like 'I walked into the attic'. Then in the second sentence, completely turn it around with something unexpected and terrifying such as 'The attic was filled with hundreds of dolls, all with their eyes following my every move'. Also, use strong adjectives to enhance the horror, like 'eerily', 'terrifyingly', etc.
I found a doll in the attic. Its eyes seemed to follow me as I left the room.
Example 1: I heard a noise downstairs. When I checked, there was no one there, but the front door was wide open. Example 2: She woke up in the middle of the night. A dark figure was standing at the foot of her bed.
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.
I walked into the empty room. Suddenly, I felt a cold hand on my shoulder, but no one was there.
There was a knock on the door. Nobody was there when I opened it.
I woke up to find a shadow at the end of my bed. When I turned on the light, it was gone.
One: 'I woke up to find my reflection missing from the mirror. And I heard footsteps coming from the empty room.'
Well, you could think of a common fear and build on it. Like, 'I heard footsteps in the attic. When I went to check, the door slammed shut behind me.' The key is to create a sense of dread quickly.
The scariest elements often include the unknown. For example, in stories where there is something sensed but not seen clearly. Like 'I felt a presence in the dark corner. When I turned on the light, the presence was gone but a cold draft remained.' Here, the unknown presence is very frightening.