Regarding the hidden basement story, the scariest aspect is the strange symbols on the walls. They seem to hold some sort of dark meaning. For the old oak tree story, the appearance of the crying child figure is really spooky. It's an unexpected and eerie sight. In the piano story, the fact that it plays by itself, as if it has a mind of its own, is quite frightening.
In some of the stories, the sudden appearance of the supernatural entities is the scariest part. Like in the haunted house story, when a ghostly figure just pops up in front of you out of nowhere. It gives you an instant shock.
The sudden appearance of unexplained figures. For example, seeing a ghostly child in the seat when you know there's no one there. It gives you a shock.
For me, the scariest part is when the driver found the claw mark. It just gives this sense of something unknown and potentially dangerous being on the bus.
The invasion of privacy in smart home device hacks is very scary. Knowing that someone could be watching you through your own camera or messing with your home environment without your permission is a huge concern. It makes you feel vulnerable in your own home, which is supposed to be a safe place.
The sudden appearance of something strange. In the story of the face at the window, the babysitter is just going about her job, and then suddenly there's this creepy face. There's no warning, and it's something that seems out of place and threatening, which is really terrifying.
The feeling of helplessness. For example, in a haunted house story, when you're trapped inside and there's no way to escape the terrifying presence. You can't fight it because you don't know what it is. It makes the story really scary as hell.
The suddenness is often the scariest part. Lightning can strike without warning, like in the story of the hiker where it hit the tree near him so suddenly. Another scary aspect is the power it shows. When it causes destruction, like splitting a tree or hitting a chimney and causing sparks. It makes people realize how vulnerable they are.
For me, the scariest is the voices. In exorcism stories, the possessed often speak in strange voices that are not their own, sometimes multiple voices at once. These voices can be guttural, demonic-sounding, and they often say things that are disturbing or full of hatred.
The sense of isolation. Orphans are already alone in the world, and when something scary happens, they have no real family to turn to. For example, in the haunted orphanage stories, they can't just go home to a safe place.
The unexpectedness. For example, in a story about a spooky mirror, you might think it's just a normal mirror at first. But then suddenly it shows a face that's not yours, and that sudden shock is a really scary part. It's the element of surprise that makes dumb scary stories effective.