God of War/Challenge of Hades

Muse Keys
Defeat the enemies on your way back to the Rings of Pandora from the Challenge of Poseidon. Once you've done that and you're given entrance through the previously blocked passage, make a left turn and head through the first opening you see to your left. Press psx: R2 when you approach the door here and you'll be asked if you want to insert the Muse Keys, which you should have by this point. Select "Yes" and the door will lower. After opening the experience chest to the left, make your way toward the faces on the wall. Get close enough to both and Zeus bestows upon Kratos the gift of health and the gift of magic, extending your health and magic bars no matter how many Gorgon Eyes or Phoenix Feathers you have. Going to the center gives you the gift of experience, and while Zeus doesn't give you enough to upgrade all of your powers, it could be the bunch of experience that gives you the boost you need. Take the time to upgrade some of your weapons and abilities, and then continue back to the Rings of Pandora.

Preliminary Sacrifices
Use the lever next to the room you were just in to turn the rings past two rooms, then go further into the center of the rings, up the stairs and into the pool. Now that you've completed the Challenge of Poseidon, you have the ability to dive down into the pool and use the small underwater passage down below. Follow the twists and turns of the passage until you reach the surface. The narrow hallway will lead you to a large room illuminated by fire. Once you enter the room, the game stops you and points out a necessary lever. Break the wall to the right of the room and follow the path past the save point and up the ladder to begin the Challenge of Hades. Go up to the door on the left and press psx: R2 to find out the requirement for opening the door. As it turns out, Hades challenges you to open the door by making some blood sacrifices. Accept the challenge and a pack of centaurs will come running out to greet you. While it seems that every new enemy introduced is more vicious than the last, the centaurs, even in a pack, are not as challenging to defeat as they look. Head to the circle set up for you back down by the entrance and get ready to make some sacrifices. Use Poseidon's Rage and Medusa's Gaze to take out the centaurs quickly, but make sure you kill them in the circle. Kill four and the door you accepted the challenge for will open, but only a bit. Head to the other sacrifice circle on the other side of the (red) pool and do the same to get the door open completely.