15 years have passed since the defeat of the Genma and the evil lord Nobunaga Oda. Darkness that once plagued the lands slowly crept back to their shadowy depths, leaving behind nothing but a whisper of their former self.
Hideyoshi Toyotomi, Nobunaga's successor became the new ruler of Japan and all seemed peaceful. Until now...
With the rise of an eerie crimson star to the East, Hideyoshi underwent an evil transformation, spreading malice and cruelty to all lands, near and far. The Genma, hearing the call of evil, started to reappear in the land, striking fear into the hearts of the people.
source: Onimusha: Dawn of Dreams