A brief description of the Dungeon Meshi manga:
It all started in a small village. One day the villagers heard moans coming from the mausoleum. They opened it and found a man there. That man was king more than a thousand years ago, now fallen, the golden kingdom, but the kingdom, once famous for its riches, was not just destroyed by the power of a mad wizard, it went underground, “To him who overcomes the wizard, I will give as a reward my kingdom” – These were the last words of the former king.