Once you give Matouya's medicine to the Elf King, you'll get your hands on a key that gives you access to all kinds of cool treasures. Here are all the areas where you can use the Shinpi no Kagi (Mysterious Key):
In Elfheim, walk to the top of the town where all the gravestones are. Click the one to the left and it will say, "Here lies Link." Some of you may be familiar with the name.
This stone reads "Here Lies Erdrick" in some versions of the game, a reference to Dragon Quest/Warrior.
Addition by Kilmar