Well, I believe I'm the one who told you that in the Bondage Kirito quest. According to the legend, When Emperor Sutoku died, he got a lot of resentment against the Imperial court and became a demon, specifically the Dai Tengu, and contributed a lot in the fall of Imperial Japan court along with rise of daimyo/samurai. In other word, the one who started Sengoku Era.
We could always replace Dai Tengu with Nurarihyon, its the other variation. And there are even a variation that said it was FOUR great youkai, and include both Nurarihyon and Dai Tengu.