Hazure Skill: The Guild Member with a Worthless Skill Is Actually a Legendary Assassin, Vol. 3 LN

Roland, the world's greatest assassin, has killed the greatest demon lord that's ever lived, but instead of living out the rest of his days as a wealthy and famous man he chooses an exceedingly normal life working at an adventurer's guild.
AN OLD FRIEND AND A BRUTAL SECRET​Every adventurer at the Lahti branch is abuzz with excitement over the discovery of a new cavern. Roland, the former legendary assassin, is chosen to lead the investigation of the underground area. The hollow turns out to be far more than just some natural formation where questers can pick up rare materials and treasures, however. A passage within leads to an arena where the elite gamble on brutal death matches. What’s more, one of Roland’s former companions is a fighter! Saving a friend and dismantling a secret gambling den sounds like something a perfectly normal guild employee would do...right?