Ochikobore Eji Minarai No Shounen. (Jitsu Wa) Saikyou Saiaku No Asashin.

In a certain kingdoms capital, war is approaching. A young boy named Il, an apprentice guard, is treated as a useless person who cant do his job properly, living a lazy life while constantly relying on his stepsister. Amidst this, a certain rumor is spreading throughout the city. It is said that a mysterious group of assassins known as the Night Dwellers are operating in the shadows. This is the story of the worst assassin among them.