Hazure Skill: The Guild Member with a Worthless Skill Is Actually a Legendary Assassin, Vol. 6 (Light Novel) (novel)

Roland's victory over Amy came at a high price, and Rila's collar was destroyed in the process. Hoping to repair the powerful item, the former assassin searches for its creator, which takes him to an old friend. Meanwhile, tragedy strikes in the foreign nation of Reubens when the king is found dead.

Only one of the greatest assassins in the world could have done this. It almost seems like Roland himself was responsible...