Fantasy: Only Monsters Can Kill Monsters

I don't know when the world began to have existences like monsters. Fortunately, humans have learned to practice, but compared to monsters, humans are ultimately at a disadvantage. Except... Except for a man who is more of a monster than a monster. While his peers were still studying and practicing in various martial arts academies, he was already killing monsters on battlefields outside the territory... This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.