Hero Overpowered but Overly Cautious Novel Sc Vol 07

(W) Light Tuchihi (A) Saori Toyota Feeling ill-prepared for the looming battle against Mash the Dragon King, Seiya returns to the underworld to acquire some new skills. But every boon gained from this dark realm comes at a cost. While Seiya practices a stealth ability, Rista turns invisible! As he hones the power of darkness, disaster strikes! Will Seiya be able to master these new techniques before the drawbacks wipe out his party?!