Every 3,000 years the realm of humans (Ninjinkai) is united with the realms of the demons (Makai) and the man-beasts (Juujinkai) by the revival of the Overfiend (the Choujin), the god of gods. The saga follows Amanojaku - a man-beast - and his quest to find the real Overfiend and to ensure the safe future of all three realms. However, he finds his beliefs put to the test when he encounters numerous Makai, who are out to destroy the Overfiend and prevent the three realms from joining.

(Source: Wikipedia)