The Crimson war is the battle of humans against demons. The country of "Alkenia" where the the humans resides and the country of demons "Onigui".

Follow the life of our protagonist Shiragaki Natsuo who lost everything that is important to him at a young age He blames himself for not being able to protect the things that are precious to him in which he sought power. prior to the tragedy he discovers a mysterious sword with a crimson red glow within a cave.

Will this sword be able to give the power that he seeks to protect or will it lead to something more ominous?.