In Japanese folklore, what demon is a giant spider yōkai that lived in mountains? The answer is Tsuchigumo.
Tsuchigumo literally means “earth spider” or “ground spider.” In Japanese folklore from the medieval period onward, it became the name of a monstrous spider-like yōkai, often imagined as a huge creature hiding in remote mountains, caves, or abandoned places.
The legend is strongly associated with Minamoto no Yorimitsu, also known as Raikō. In versions connected with The Tale of the Heike, Yorimitsu battles a spider monster and gains fame for defeating it. The fourteenth-century picture scroll Tsuchigumo Sōshi gives the story a particularly macabre form: a trail of blood leads warriors to a colossal spider cave filled with skulls and smaller spiders.
The name has an older and more complicated history. Early Japanese chronicles used tsuchigumo for certain rebellious or independent local groups, not necessarily literal spiders. The supernatural spider interpretation developed later. It is commonly confused with Jorōgumo, another spider yōkai that transforms into a seductive woman, but Tsuchigumo is the better match for the giant mountain-dwelling spider in this question.