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.
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'.
Well, start with a scary setting or a mysterious character. Like, 'The old house creaked. A shadow moved behind the curtains.' Make it short and spine-chilling. Also, use simple but powerful words to build tension quickly.
Think of a common fear and describe it in an unusual way. Say, 'The mirror showed my reflection. But it wasn't mine.' Keep it short and intense.
First, come up with a terrifying idea. Then, in the first sentence, set the stage and hint at the horror. In the second sentence, reveal the full horror in a sudden and unexpected way.
Start with a creepy setting or situation. Then, end with a shocking twist. That's the key to a good two-sentence horror story.
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.
The sense of the unknown also plays a big role. Consider 'I heard strange noises from the attic. When I opened it, only darkness stared back at me.' We don't know what's in the attic which is terrifying.
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.