This illness was probably real for some people, but because others couldn’t see it, they were dismissed as liars. They treated them like a disgrace and tried to erase them from history. Now, the forgotten ancestors who once suffered from it are coming back, taking the form of the monsters from their own hallucinations, revealing themselves to the townspeople and forcing them to relive their experiences, just so they can no longer be ignored
It started from hallucinations. But then people psy-oped themselves into becoming actual spooks after death and it started sort of a self-perpetrating cycle.