Nabelle , Sarah And Angelic are children of the reincarnated hero Jeanne Arc which resides in the Bailian Empire. Their mother controls a town called Baili where heroes are summoned by 50 members of the royal faction during times of war. She control a cave that cannot be destroyed by in a physical sense/means but countless mineral known within their world is endless within that cave so it's commonly mined and paid is raised based on the length of mining.