One could be: 'I woke up to the sound of breathing beside me. It wasn't mine.' Simple but creepy.
A common one might be: 'I saw my reflection smile back. I wasn't smiling.' This short story gives a sudden chill.
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.
I heard strange noises coming from the attic. When I opened it, a pair of yellow eyes stared back at me.
I found an old diary in the attic. The last entry said 'They're coming for me, and now they know I'm here' and then the pages started to bleed.
The little girl found a doll in the forest. As she picked it up, the doll's eyes started to bleed.
A classic story example could be 'Cinderella'. It's a well-known fairy tale where a poor girl overcomes hardship and finds true love.
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.
Isolation is another factor. Consider 'I was stranded in the old house. I felt eyes on me from every corner.' Being alone in a spooky place and feeling watched, even though there's no visible source, is a classic horror trope that makes the story very creepy. It makes the reader feel the character's vulnerability and fear.
Well, here's one: 'I woke up to a strange noise. When I turned on the light, no one was there.'
It should have a sudden twist and create a sense of dread in just two sentences. For example, 'I woke up to a knock on the door. No one lives next door.'
Story 1: I woke up to a cold hand on my face. When I turned on the light, no one was there.