The world's finest assassin gets reincarnated in another world as an aristocrat. 3 - Rui Tsukiyo

To Kill That Which Cannot DieAfter 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...