Each time you defeat a fiend it will unlock a secret dungeon. There are four of them and they include boss battles from other FF games.

• Defeat Kraken near the canal by Melmond to unlock Lifespring Grotto.
• Defeat Lich near the Temple of Chaos to unlock Earthgift Shrine.
• Defeat Marilith near the Dragon Islands to unlock Hellfire Chasm.
• Defeat Tiamat in the rivers by the Ice Cave to unlock Whisperwind Cove.