How can 'tuck me in two line horror stories' be so scary?

One could be: 'I tucked myself in. Then I saw a face under my bed.'

I opened the basement door. A hand reached out from the darkness and grabbed my ankle.

They are effective by creating immediate tension. For example, 'The phone rang at midnight. A raspy voice said 'I'm coming'.", "Answer2":": 'Scary two line stories' work well because they rely on the reader's imagination. In a story like 'The moon was full. A howl that wasn't from a dog echoed.', the reader has to fill in the blanks about what might be making that howl. It's this sense of mystery that makes them effective.

Well, one possible 'scary two - line story' could be 'I heard a knock on the door at midnight. When I opened it, no one was there but I felt a cold hand on my shoulder.' Another could be 'The old house was empty. As I walked in, I saw a shadow that moved on its own.' And a third one might be 'She walked through the graveyard. Suddenly, a whisper called her name from an unmarked grave.'

I woke up in the middle of the night. There was a figure at the foot of my bed, staring at me with empty eyes.

They're short and to the point. Just two lines can quickly set a spooky mood.

Using vivid and spooky words. For example, instead of 'a figure', say 'a shadowy, hunched figure'.

I woke up in the middle of the night. There was a shadowy figure at the foot of my bed.

I woke up to find a handprint on the mirror, but I live alone.

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.