The Mystic KEY

Once you've obtained the CRYSTAL, the hard part of the mystic KEY quest is over. Head back to ElfLand to rest, then hop on your ship and head north. There is a dock north of Coneria which is the most convenient place to disembark on your journey to Matoya's Cave.

Matoya's Cave

If you haven't visited Matoya's Cave before, make sure to open the chests here. Speak to Matoya to trade the CRYSTAL for the HERB. Now it's time to return your ship and sail right back to ElfLand.

Castle of Elf

Speak to the Prince's attendant to wake him up with the HERB. He will give you the mystic KEY, which allows you to open treasure rooms in Castle of Elf, the Northwest Castle, Marsh Cave B3, the Temple of Fiends, Castle Coneria, and the Dwarf Cave.

You can find the treasure room in the castle by returning to the entrance and heading east, hugging the outer wall as it turns north. You'll find 730 G, a Copper Gauntlet for your FIGHTER, and a nice Silver Hammer for your Wh.MAGE.