I think 'Looked in the mirror. Saw someone else.' qualifies as one of the shortest. It gives that sudden shock in a brief sentence.
Regarding 'She opened her eyes. Darkness everywhere. Then she realized she was already dead.', it's the sudden realization of a terrible truth. The contrast between thinking you're alive and then finding out you're dead is a huge shock. It plays on our fear of the afterlife, of the unknown state of being dead, and the fact that we may not even be aware of our own death.
The shortest short story ever written could be something like 'For sale: baby shoes, never worn.' It's extremely concise but conveys a poignant situation.
There are various contenders for the title. But a commonly mentioned short one is 'The Last Man on Earth Sat Alone in a Room. A Knock Came on the Door.' It packs a punch in its brevity.
Atmosphere. A good shortest scary story can quickly set a spooky mood. Using words that imply darkness, isolation or the unknown. Like in the story where a girl is in an old house and hears a whisper. The old house, the slamming door and the lights going out all build a terrifying atmosphere.
The definition of the 'shortest novel' can be quite subjective. Different people might consider different works as the shortest. Some very concise novels might focus on a single intense moment or a simple plotline.
It can be just a few words. Some really short stories are like a single sentence or a very brief paragraph.
The brevity. They leave a lot to the imagination. For example, in 'The phone rang in the empty house', we don't know who's calling or what will happen. It's the unknown that scares us.
They are often concise. For example, they can quickly create a sense of unease with just a few words, like 'The doll's eyes followed me.' It doesn't need a lot of description to be scary. The simplicity makes the horror more immediate.
The shortest scary story could be 'The last man on Earth sat alone in a room. There was a knock on the door.' It's short but very effective in creating a sense of fear. Just think about it, if you were the last person, who or what could be knocking?
The shortest published novel can vary depending on different criteria. Some might be just a few pages, possibly exploring a simple yet profound idea in a concentrated way. It could be due to the author's intent to be highly concise and impactful.