Born in 1607 as the eldest son of Yagyu Munenori, master of the Yagyu Shinkage-ryu school and sword instructor to the Tokugawa shoguns. He showed exceptional talent in swordsmanship from childhood. He lost the sight in his right eye at a young age, though the cause remains debated — theories range from a training accident with his father to a hawking injury to a congenital condition. Around age 9, he entered the service of the third shogun Tokugawa Iemitsu, but in 1631, at age 24, he abruptly departed from Iemitsu's court and vanished from official records for 12 years. These "missing years" spawned countless legends: some say he traveled Japan as a secret agent (onmitsu) for the shogunate, others that he went on a warrior pilgrimage, and still others that he was confined due to a falling-out with his father. He resurfaced in 1643 and is said to have served the lord of Odawara. He authored the martial arts treatise Tsuki no Sho ("Notes on the Moon"), which systematized Yagyu Shinkage-ryu theory, particularly the concept of ken-tai icchi — the unity of offense and defense. In 1650, he collapsed suddenly during a hawking expedition and died at age 44. The cause of death is unclear; stroke, assassination, and chronic illness have all been proposed. His enigmatic life has made him arguably the most fictionalized samurai in Japanese popular culture, appearing in countless novels, films, manga, and video games.