The Hero Is Overpowered But Overly Cautious, Vol. 7 (light Novel)

"Feeling ill-prepared for the looming battle against his former pupil, Seiya returns to the underworld to acquire some new skills. But every boon gained from this realm comes at a cost--Rista's dignity. First, his group hones their stealth abilities by learning invisibility magic. Then, Seiya ventures to the Infinite Corridor to harness the powers of darkness! They'll need more than a couple new tricks to defeat Mash, the Dragon Lord, though. To gain an edge in the fight to come, they will have to peer into the past and learn the tragic origins of Warped Gaeabrande..."--Publisher's description.