World introduction: On a night in 2013, a meteor streaked across the sky, containing many small creatures that broke free upon contact with oxygen and drifted onto the sea surface.
These small creatures, the size of a thumb and shaped like translucent little snakes, possess an exceptional survival ability in the sea, with some lucky ones drifting onto land.
Upon reaching land, these small creatures revealed their terrifying side, using their small size to seek intelligent creatures to parasitize.
Humans were their primary targets; they would first burrow into the human body, head straight for the brain, devour the entire human head, grow on the nutrients consumed, and finally disguise themselves as a human head, parasitizing on a headless body, possessing thinking abilities, blending in with ordinary people, indistinguishable from normal humans, known as Parasitic Beasts.
