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

To Kill That Which Cannot Die​After his success in battle with a demon, Lugh is made a Holy Knight by the Alvanian Kingdom and tasked with slaying more of the creatures. The problem is, only the hero can actually fell demons. Such a setback isn’t going to stop the world’s greatest assassin, however, and Lugh gets to work devising a spell to kill the supposedly immortal beings. Unfortunately, the only way to test this new magic is in a real battle! Failure promises certain doom, but triumph may turn out to be just as perilous if Lugh isn’t careful...