Lord of Nihonmatsu Castle in Mutsu Province, a descendant of the Nihonmatsu Hatakeyama who ruled Adachi district in Mutsu. As Date Terumune—Masamune's father—progressively subjugated the daimyo of Oshu one by one, the Nihonmatsu domain was steadily encroached upon, and Yoshitsugu accumulated deep resentment and humiliation. In October 1585 (Tensho 13), when Date Terumune visited Nihonmatsu Castle to mediate a territorial dispute and was about to depart, Yoshitsugu suddenly seized him under the pretense of saying farewell and abducted him (the Awanokasu Incident). He fled toward Awanokasu in Adachi District with Terumune as a hostage, but was cornered by pursuit forces acting on Date Masamune's orders; the entire party, including Yoshitsugu, was cut down at the Abukuma River. Whether Masamune truly issued the order 'kill them both, my father included' has been debated since early times and has not been definitively settled among historians. The incident prompted Masamune to push the siege of Nihonmatsu Castle with even greater vigor, accelerating his drive toward the unification of Oshu. Yoshitsugu died at thirty-three.

A warlord who endured years of Date pressure until he finally snapped. Like a cornered rat, he made a last desperate gamble—only to fall before Masamune's merciless resolve. He embodies the difficulty of survival for a small lord in the Sengoku era.

The Awanokasu Incident is remembered as one of the most dramatic events in Date Masamune's story. Terumune's death brought deep grief to Masamune while also providing justification for subsequent campaigns. Historic sites in Nihonmatsu City preserve this history.