The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat, Vol. 3 LN

After Lugh and Epona's victory over the demons who attacked the Royal Knights Academy, House Tuatha Dé is charged with subduing the monsters. Though demons are supposedly immortal, Lugh combines his own knowledge with a new power bequeathed to him by Epona to develop a way of taking them out. "There's nothing I can't kill. I'll make quick work of you!"