nurikabe

Nurikabe , hiragana: ぬりかべ , a demon or yōkai from Japanese mythology and part of folklore on the Island of Kyushu.

Manifesting as a white or invisible wall with arms and legs blocking the path, it obstructs and misdirects walking travellers at night, blocking roads and delaying the journey . Trying to walk around it is futile- it extends itself eternally, blocking those trying to avoid it or running away, it may even fall on and crush the traveller. Many- sensing the invisible barrier around them- instinctively try to navigate around him and find themselves more lost than before or taking a longer route.

Some tales relate that Nurikabe is sticky, and upon contact the victim is unable to extricate himself, becoming part of the wall.

Only knocking on the lower part of the wall with a stick will make it vanish.

Also found in modern manga and videogames (the inspiration for the Whomp in Mario Brothers) and the namesake of a binary determination puzzle.