The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat, Vol. 7 (manga) - Hamao Sumeragi - Bog

Thanks to his valiant efforts in defeating the orc general, Lugh is appointed as a Holy Knight. However, no glory comes without strings attached, as he is now tasked to subjugate the demons only the hero can kill. And to do so, Lugh must ensure his and the girls' abilities are up to par - especially with the appearance of a new demon!