The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat, Vol. 7 LN by Rui Tsukiyo

Naoise has turned his back on humanity and become a servant to Mina, the snake demon. Lugh thought he could find a way to stop his wayward friend without killing him, but things come to a head when Naoise turns his sword against his own people. The world's finest assassin has slain wicked aristocrats and monstrous creatures alike, but this is an opponent unlike any other-one he cares about.
Binding: Paperback / softback