In a far future version of Earth, ordinary creatures have been twisted into monstrous beings known as aetherborn, creatures of legend and nightmare that terrorize what remains of humanity. The only effective weapons against these beasts are mythical items called aetherial arms, which mysteriously appeared around the same time as the monsters they are meant to fight.

Crow Titus was a notorious layabout who never wanted to be a hero, but his quiet life ended when a dark magus using a powerful aetherial arm led an attack that destroyed his village, leaving him the sole survivor. In the aftermath, Crow stumbles upon a cursed sword named Muramasa, one of the aetherial arms. However, he does not so much wield the weapon as it wields him. Muramasa takes control of Crow’s body like a puppet, forcing him to fight in order to feed its insatiable hunger for souls, though it fortunately prefers to feast on the wicked rather than the innocent.

Desperate to keep his curse hidden and simply survive, Crow finds himself trapped in a frustrating charade. While he is terrified of being controlled and potentially becoming a mass murderer, everyone else mistakes his unwilling actions for those of a gallant and powerful hero. His attempts to convince people otherwise fail miserably, and his lies and frantic justifications only deepen the misunderstanding, earning him even more respect and adoration as a so-called myth conqueror someone who can freely command cursed weapons. His newfound magus knight mentor, Iris, further complicates his life with her persistent advances.

As Crow tries to play the part of a reluctant hero while battling Muramasa’s madness, his escalating reputation lands him in increasingly dangerous situations. He attracts the attention of an evil organization called Wanpurgis. The king himself eventually orders Crow to hunt down and kill a dragon, the most powerful kind of aetherborn, alone. Armed with his cursed blade that may actually be armed with him, along with a growing collection of other troublesome weapons, Crow is forced to embark on this seemingly impossible mission, relying on a sword that could turn him into a monster at any moment.