Descriptive sentences help to bring the setting to life. In a scary story, if you say 'The abandoned asylum was a hulking mass of decay. Windows were broken, and the rooms were filled with rusted medical equipment.' It gives a vivid image in the reader's mind and adds to the scariness. The details make the place seem real and more threatening. Also, when you describe characters like 'His eyes were sunken pits, and his skin was a sickly gray as if the life had been drained out of him.', it makes the character more menacing and spooky.
The moon was a pale, sickly orb, casting long, distorted shadows that seemed to writhe like living things. The wind howled through the bare branches of the ancient trees, sounding like the wails of the damned.
One more for you. 'She walked into the empty basement. A strange whisper told her she would never leave.' The basement is often a place associated with the unknown and danger in horror stories. The sudden whisper indicating that she's trapped is very scary.
A good one is 'The doll's eyes followed me. Then it smiled.' Dolls can be really creepy, and the thought of its eyes following and then it smiling is quite unnerving.
Make the sentences vivid. Don't just say 'It was scary.' Instead, say something like 'The walls seemed to ooze a dark liquid that smelled of decay, and the air was filled with a deathly silence.' Vivid descriptions make the horror more palpable to the reader.
The unknown. When there are things that are not fully explained or shown, it scares people. For example, in a haunted house story, if you just hear strange noises but don't see the source clearly, it creates a sense of fear. You start imagining all sorts of terrifying things.
What makes a scary story truly scary is the element of the unknown. When there are things that are left to the imagination, it scares people more. For example, in a story where there's a strange noise in the attic but you don't know what's causing it. It could be anything, and that uncertainty is terrifying.