I'm Quitting Heroing, Vol. 6 (manga)

Former hero Leo and his friends in the Demon Queen’s army head back to the demon realm, only to find a terribly polluted environment. At this rate, the demon realm is sure to be destroyed in a year…so it’s time for Leo to roll up his sleeves and get to work on saving the land! Meanwhile, the Demon Queen Echidna has a problem of her own to take care of…?!