Patients

Evil spirits from rumors were utilized as tools.

The first-generation Demon Slayers were, in many senses, legendary beings as terrifying and powerful as evil spirits themselves.

After several generations of improvements, the Sanity Value test evolved into two forms: written and physical. The former assessed mental state, requiring only that one sit at a table, answer some questions, and then wait for an evaluation. The latter determined the threshold of Sanity Value, necessitating contact with those bizarre entities.

Most existing Demon Slayer organizations adopted the former method, needing only to ensure the mental state of new members was stable.

The latter was common among tightly knit organizations, such as the Night Watch.

"You already know what Sanity Value is used for, now for the second thing I need to tell you: why Sanity Value is needed."