The legend that Kukai secluded himself in the Mikurodo caves at Muroto-misaki Cape to complete Kokuzo Gumonji-ho is well known, and Nichiren also worshiped Kokuzo Bosatsu for 21 days when he started to follow the teachings of Buddha at the age of twelve.
空海が室戸岬の洞窟御厨人窟に籠もって虚空蔵求聞持法を修したという伝説はよく知られており、日蓮もまた12歳の時、仏道を志すにあたって虚空蔵菩薩に21日間の祈願を行ったという。