There are two Muse Keys hidden in the game, which are used to unlock a door in the Rings of Pandora. One is hidden in the underwater passage, after you've opened Poseidon's door; there will be a breakable wall that you can go through to reach an area with several enemies, breakable pots, and a chest containing the first key. The second key is found while going through the challenge of Poseidon, at the top of the ledges you must climb; simply go right when you've reached the top to find a chest with the second key. Once you have both keys, you can enter the room to obtain two Red Soul chests, and three fountains that will increase your maximum health, magic, and will give you five bars of Red Souls, respectively.