Mochitoyo Yamana became a priest under the name of Sozen. During the Kakitsu Incident (the revolt of Akamatsu against Shogunate) he occupied Shirohata Castle and annexed Akamatsu's tenitory (1441). Later he conspired with Katsumoto Hosokawa to make himself an important official in the Hatakeyama family. As soon as the Hatakeyama family began to decline he fell out with Katsumoto and began conspiring with Shogun Yoshimasa Ashikaga's wife to, make her son, Yoshihisa, Shogun in place of Yoshimi, the adopted son of Shogun Yoshimasa. This led to the Onin Civil War (1467). During the war Mochitoyo died.