According to Otogizōshi stories compiled several centuries later, Sadamitsu was a retainer of the Japanese legendary hero Minamoto no Raikō.
He received a prophecy saying, "I am touched by the sincerity of your chanting and will grant you a sacred spring that will cure the illnesses of 40,000 people.
On another occasion, when Sadamitsu returned home, he found a giant poisonous snake living in Usui Pass, tormenting the people.
With the protection of the Eleven-Headed Kannon Bodhisattva Ekādaśamukha, Sadamitsu swung his scythe and defeated the snake.
He then built the Usui-san Joko-in Kongo-ji Temple and enshrined the Kannon Bodhisattva and the snake's skull there.