18+ Yamete Kure, Tsuyoi no wa Ore ja Nakute Ken nanda......!

Crow Titus, notorious layabout, wasn't prepared for the destruction of his village by the aetherborn. On Earth, far in the future, ordinary creatures have been twisted into the monstrous aetherborn—beasts of legend and nightmare who wreak havoc on humanity. The only defenses against the creatures are mythical weapons known as aetherial arms, which, luckily, appeared right around when the monsters did. Less luckily, some humans choose to use those weapons for evil—for example, the attack on Crow's village was instigated by a dark magus wielding one. The lone survivor of the massacre, Crow somehow stumbles upon an aethereal arm, Muramasa. Sword in hand, Crow decides to fight back—or, well, Crow can't decide much at all. Muramasa has taken control and manipulates Crow like a puppet, using the man's body to feed his insatiable appetite for souls! Can Crow play the part of a hero while surviving Muramasa's madness? What about surviving the advances of Iris, his newfound magus knight mentor?

(Source: J-Novel Club)