Nurikabe

The nurikabe is a youkai, or spirit, from Japanese folklore. It manifests as a wall that impedes or misdirects walking travelers at night. Nurikabe's main ability is that it can change its form and expand itself.

Trying to go around is futile as it extends itself forever, but knocking on the lower left part of the wall makes it disappear. It has been suggested that the legend was created to explain travellers losing their bearings on long journeys.