In the eschatology of the Kuei-Jin, the Sixth Age will be marked by the arrival of a supreme lord of darkness and horror who will rule over the world with an iron (and spiky) fist. This coming Demon Emperor will be the hallmark of that fallen age, and the only mercy in his time is that the Sixth Age will be brief...if the Universe is still running properly.
Guessing who will be Demon Emperor is a popular sport for Kuei-Jin and White Wolf players alike. The Yama Kings generally assume that the Demon Emperor will be one of their number, with the most common candidate being Mikaboshi. Other common choices include Saulot, who seems to have encountered the mythology when traveling in the East and decided that ruling as Demon Emperor would be preferable to reapproachment with the Almighty.