Mount Sagiri

Sagiri Mountain (狭霧山, Sagiri Yama) is a huge mountain in a mostly unpopulated region. The air is very thin near its peak, making it very difficult to breathe.

History
The mountain is used by Urokodaki who uses the mountain to train children who are craving to become a Demon Slayer. Urokodaki tests his students by tasking them by slicing a bolder using a katana and if they succeded then they are sent to participate in the Final Selection. Children who complete the Final Selection are accepted as Demon Slayers. For those who die in the Final Selection, their spirits return to the mountain.