The story takes us back in time of Nobunaga, Hideyoshi and Tokugawa era. People strived for the top using many kinds of word. It was believed that human’s strength is no longer sufficient to get power. Genma, the demons, are used as tool to win the battle. They’re mean and heartless. The priests controled Genma as assigned from the higher ups.
Those who fight Genma are called onimusha. They have extraordinary power to vanish the evil spirits.
Onimusha is a great manga, we can recall many historical celebrities such as Tokugawa Ieyasu, Hattori Hanzo, Juubei Yagyu, Mushashi (Miyamoto) and many more. You’ll love it. The art is beautiful and clear.
Bad spirits and dark magic have been rooting since long time ago, long before we became pitechan tropus erectus. And like the other evil practice (like prostitution and gambling), it continues to operate. But, now they have different names on each.
It’s hard to imagine that war require as many as possible participant, whether it is human or not. To strive for power, Nobunaga, Hideyoshi and Tokugawa fought a lot. I heard that magical practice is necessary. Shaman (magical priest) must be a good career back then.
Talking about Shaman (magic priest), there is a well known priest character called Abe no Seimei who had great power of predicting future, and also manipulating creatures around him. His power was used to help the kingdom. I believe this character has been shown in some other mangas.