The Sendfang 13th Street Haunting

Crow's-eye fruit, when swallowed, lets one see various demons.

Grind it into powder, mix it with sweat, and put it in the eyes. At night, you can see ghosts.

The blurb comes from an old edition of the Compendium of Materia Medica. Modern supernatural fiction, Chinese-style horror, and an infinite-loop setting.

I'll leave this here and write it later.

Theme: Think it through before settling on one.