One thousand years ago, hell invaded the earth in an event called the demonic conversion event. As humanity tried to overcome their demonic counterparts, three people came together to help humanity push back their ghoulish invaders. Being called the Trinity saviors, they used their powers to create objects called akuma no fujikome to seal away the demons. One of these objects was a sword called the sword of legion. With this sword, they were able to seal away one-third of hell's army inside it. Putting an end to the demonic conversion event and protecting humanity and allowing it to live the other day.

After almost a thousand years of peace with no major demonic activity, a group of people use a ritual to bond the sword to a newborn baby. Now all grown up, the young man cursed with the sword travels the land of Tomani doing mercenary work for a living as a samurai, as the negative aura surrounding the sword prevents him from living a normal life. While he fulfills his seemingly only purpose in life, he longs for a way to get rid of the curse affecting him, so he can see what it is like to live as a human and not a monster among men. But with no answers to the people who bonded him to the sword, it seems more likely he will live his life as a monster, only longing for normality in his life.

Known as the infamous Blood Samurai for his brutal killing methods, Kiyoshi Kataigure must choose what path he wants to live before falling into darkness and triggering other demonic conversion event.