What's the most frightening thing about your world?

Pretty much what the title says. Describe something that is super creepy, or scary about your world. It could be something related to your magic system, an environmental aspect (like weather), the consequences of human actions, or whatever. So long as it's something that would cause fear in the dwellers of the world.<br>Another thing you can include if you like is the attitude that inhabitants have towards this concept, and how they hold it in their regard. Do they fear it, revere it, worship it? How does it influence their behaviour and daily lives?