In Chapter 14, when you go to the dungeon to fight the boss, nothing seems to happen. Run through the bottom section, go back to the dungeon map, and run around to the top entrance. Then, go all the way south through the dungeon again and there will be a switch sitting in the corner. Find and flip it. A gate will open, allowing you to return to the initial area. Go all the way southeast (backtracking through previous areas) and the boss will appear.