In 1392, after his stronghold Chihaya-jo Castle was fallen by the army of Muromachi bakufu (Japanese feudal government headed by a shogun) led by Motokuni HATAKEYAMA, Masamoto, aiming for turnaround, conspired to assassinate Shogun Yoshimitsu ASHIKAGA, but he failed and was executed the next day.
1392年、畠山基国が率いる室町幕府軍によって本拠千早城が落とされた後、将軍足利義満の暗殺をたくらみ逆転を狙ったが、失敗して翌日に処刑された。