険しい難所で旅人や馬が命を落とす悲劇を目の当たりにしたから。

Having become the monk Ryosei, Ichijuro set out on a journey of ascetic training across various provinces. When he reached Yabakei in the province of Buzen, he confronted an extremely steep and hazardous pass known as the 'chain crossing.' It was a place of endless tragedy, where travelers and horses lost their footing and fell to their deaths. Witnessing this devastating scene, Ryosei firmly vowed to carve a tunnel through this rocky mountain to atone for his sins. With insufficient tools, his endless challenge was about to begin.