The unknown sender. When you don't know who is sending those terrifying messages, it adds a lot of mystery and fear. It could be anyone or anything.
The sense of helplessness. You can't really stop these texts easily. Blocking the number might not work if they keep coming from new numbers. And you don't know what the sender will do next. For example, in the story of the guy whose car brakes failed after getting the texts, he didn't know how far the sender would go to harm him, and that's really terrifying.
The unknown. When you don't know what's lurking in the shadows or what's going to happen next, it creates fear. For example, in a story where there's a strange noise in the attic but you don't know what's making it.
The use of vivid and disturbing descriptions also helps. Like describing a monster with oozing sores and jagged teeth. It makes the horror more tangible. Another important aspect is the isolation of the characters. If they are alone in a spooky place, like a deserted island or an empty mansion, it intensifies the scariness.
A great horror story often plays on our deepest fears. Fear of the dark, for instance. Many horror stories use the setting of a dark, isolated place like in 'The Conjuring' series. The idea of being alone and vulnerable in the dark is terrifying. Also, the element of the supernatural can be really scary. When there are ghosts or demons as in 'Insidious', things that defy our understanding of the natural world, it gives us chills. The way the story builds tension, slowly revealing the horror, is also crucial.
One story is about a girl who kept getting text messages from an unknown number. The texts said 'I'm watching you sleep'. Every night, at exactly 3 am, the messages would come. She was so scared that she changed her number, but the new number started getting the same texts.
There was a guy who received a text that just had a picture of his own bedroom window from the outside at night. The text said 'Soon.' He was really freaked out. As days passed, he would get more texts with pictures of him in different parts of his house that he hadn't noticed anyone taking. It was a very creepy experience.
Atmosphere plays a huge role. A little horror story set in a desolate, foggy forest at night is scarier than one in a bright, sunny field. The description of the setting, like the creaking of the old trees in the wind, the cold damp air, and the silence broken only by strange sounds, all contribute to the overall scariness. And unexpected twists, like the friendly neighbor turning out to be a serial killer, can send chills down your spine.
The atmosphere. If an animation can create a really dark, gloomy and tense atmosphere, it's already halfway to being scary. For example, using dim lighting, eerie sound effects like creaking doors or howling winds.
The element of the unknown often makes a horror stort story really scary. When readers don't know what's lurking in the dark or what will happen next, it creates a sense of unease. For example, in 'The Call of Cthulhu', the Lovecraftian horror lies in the indescribable and incomprehensible nature of the ancient gods.
Isolation. When the characters are alone in a spooky place, like a deserted island or an empty mansion, it amplifies the horror. There's no one to turn to for help. It's just them against whatever evil lurks there. For instance, in a story where a person is trapped in an old castle with a vengeful spirit, the feeling of being alone makes it scarier.