Hōgen Rebellion
One notorious "tengu" from the 12th century was himself the ghost of an emperor. The "Hōgen Monogatari" tells the story of Emperor Sutoku, who was forced by his father to abandon the throne. When he later raised the Hōgen Rebellion to take back the country from Emperor Go-Shirakawa, he was defeated and exiled to Sanuki Province in Shikoku. According to legend he died in torment, having sworn to haunt the nation of Japan as a great demon, and thus became a fearsome "tengu" with long nails and eyes like a kite's.