After defeating Astos, you are now in possession of the Crystal Eye. Remember anyone needing one of these? It’s time to head back to Matoya’s Cave.
After you give the Crystal Eye to Matoya, she will give you the Jolt Tonic to wake up the sleeping Elven Prince. Now it’s time to head back across the sea, again.
Once you get back to the Elven Castle, you should talk to the elf that is taking care of the prince. She will take the Jolt Tonic and use it to wake the Prince. Once awake, he will give you the Mystic Key.
Finally! Now we can open all of the doors that were previously locked! Here is a list showing what is behind each locked door. If the item does not help your group, and you don’t feel like going back for it, then don’t. There is no prize for collecting every treasure chest in this game.