How can we develop a full horror story from 'first lines of horror stories'?

A well - known last line is 'The darkness never left.' This is very effective as it gives a feeling of eternal doom. The 'darkness' here can be symbolic of evil or the unknown horror that has now become a permanent part of the world in the story. It creates a sense of hopelessness and a chilling atmosphere that stays with the reader long after they've finished reading.

One good line could be 'The darkness seemed to breathe, a living entity that wrapped itself around me.' It creates a sense of the environment being alive and menacing.

They are effective because they are concise. For example, 'She heard a whisper. It came from inside the wall.' It gives just enough information to let your imagination run wild.

One more example is: I saw a figure in the mirror. When I turned around, it was still there but no reflection.

Sure. If the first line is 'The old house on the hill had always been there, silent and brooding.' One possible story could be about a group of teenagers who decide to explore it. As they enter, they feel a cold chill. Strange noises start to come from the attic. They find old portraits that seem to be watching them. Shadows move on their own, and one by one, they start to disappear, leaving only their screams echoing in the empty halls of the house.

The old house stood at the end of the lane, its broken windows like empty eye sockets staring into the void. This opening line creates an image of a desolate and potentially haunted place, drawing the reader in.

One great line is 'It was a dark and stormy night, and the howling wind seemed to carry the whispers of the dead.' It sets a spooky mood right away.

One could be 'I woke up in the middle of the night. There was a figure at the foot of my bed.'

The moon was full, but its light couldn't penetrate the thick fog that had swallowed the old, decrepit mansion at the end of the lane.