Ghosts.
The reason they cause fear is because they cannot be communicated with, anticipated, or understood.
In other words, they are sufficiently unknown.
Death itself, being killed by an inexplicable, beyond-comprehension being, is far more terrifying than being killed by a human criminal who can communicate but kills mercilessly.
Ultimately, the root of human fear lies in the unknown.
When something can be understood, or even just interpreted, it loses its mystery and thus the greatest source of fear.
There is once a saying — the ghost you fear might be someone else's long-lost loved one.
Regardless of its truth, it indeed greatly reduces human fear of ghosts.
It explains the incomprehensible into a concept that is universally understandable and acceptable.
Likewise, a seed was planted in Ye City at this moment.
