Hazure Skill: The Guild Member with a Worthless Skill Is Actually a Legendary Assassin, Vol. 1 (LN)

Roland is the world's greatest assassin, he's a man who's taken a worthless skill and made it so powerful that he managed to kill the greatest demon lord that's ever lived. He could live out the rest of his days as a wealthy and famous man, but instead chooses...an exceedingly normal life working at an adventurer's guild?! For someone as exceptional as Roland, aiming for a normal life won't come easy, however...