Vermillion

Vermillion (ヴァーミリオン) is a manga series by Satou Atsuki. It was serialized in Young Ace.

Plot
By using the Dark Gate, humans can entrust their hopes and needs to the darkness. The darkness they call upon is a great and evil power, known as "Demon." With the help of his partner Miroku, the Demon Slayer Kisaki Senju will slaughter every last "Demon" that is called into existence.