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 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.
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 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.
Go through the door and follow the path (don't forget to jump up on the ledge and claim the chests) which leads to another door. Lift the door up with and head through. What you'll see first is a missing block on the floor, revealing some lava which will kill Kratos if he comes into contact with it. Jump over the first block and then the block in front of it will open. Do a double jump to save yourself from Hades' cruel trap. Keep walking and you'll see a similar scene, except this time, the block which you must land on after a double jump is quickly expanding and contracting. Time your jump right to land on the floor and the lava, though this may take a few tries. Once you clear this trap, move forward and you'll see arrows being launched periodically from the walls. Dead soldiers pierced with arrows litter the scene as a reminder of how Kratos could end up, but at least the arrows, unlike the lava, don't result in instant death. Do a dodge roll past the first set, then stop at the break in the middle and do the same for the second. Proceed through the tunnels and you'll finally run into some enemies, including some archers and Medusa. Here, you'll also see a fork. Go down the left path first for some breakable pots containing experience, then the right to move on. When you see the archers, take an immediate left turn by jumping over the debris. Here you'll find a pack of centaurs. After you're finished with them, backtrack to the archers and take the lift.
Jump up, push the nearby wall in, and avoid falling into the lava for some experience chests. Backtrack, go right this time, and take another right where you should see another strange-looking wall. Push to open up a new path. Defeat the enemies, take the chest, and once again, backtrack. Keep going north, then take a right into what looks like a moving wall. When it moves enough to let you through, go in and jump on the big block. Defeat the enemies until the path opens up, and you'll end up back in the room where you defeated the centaurs for the first time.
Traps of Hades
A statue will rise out of the pool. Jump down into the pool and pull the lever at the bottom. Now climb up the statue and turn the light on the path in front of the save point. Climb up the stairs and take that path. Evade the large, molten boulders until you see a narrow path down the middle. Take it and defeat the legionaries. Open the chests, then leave. Now go back and follow the left wall until you see a handle. Lift the door and follow the path. Defeat the monsters along the way to continue down the path, and keep following it until you reach a rope. Get across it and defeat the monsters on the platform to move on. Get through the winding path and you'll reach an area where you must use the beams to get across. The initial platform leads you straight onto one. The beam goes forward and then left. Instead of continuing left, jump onto the beam to the north. Afterward, just continue along the path the beams set for you, and watch out for the turning saws along the way. Eventually, you'll reach what seems to be a dead end leading into the center of the saws. Jump onto the ladder located at the center and climb up the two ladders. Get across the beam directly in front of you, and when you reach a fork, slide down the rope to move on (the beam leads to a chest).
Jump down and follow the path. Press to turn the valve. You'll get a scene showing you that there's something behind the doors, and it's raring to get out. Of course, Kratos' job is to get to the mysterious doors and take down whatever's lurking behind them. Take the path backwards and get to the doors. You'll be locked into the room, and right on time, the beast that's been causing all the intrigue will burst through the lock and face Kratos. Here you'll be going against the 20-foot-tall servant of Hades, the Guardian. Kratos will roll away from his attacker, and the battle will begin.
Start off by hacking away at the Guardian until you get some chinks in its armor (equipping Poseidon's Rage would be useful here). Eventually, it will be overwhelmed, and you'll get a chance to do some serious damage. Press to get on the beast's back and begin a quick-time event. Move to match the onscreen prompts and, if successful, Kratos will slam the Guardian's head into the ground. As soon as this happens, run back to the staircase and pull the lever. As the log is launched at the Guardian, press again and it'll come over to face Kratos. Start hacking away at it again, and the armor covering the health bar in the corner will finally begin to fall apart. It'll keep retreating and coming back again until the health bar is fully-revealed, at which point the Guardian's bare flesh is revealed. Jump down and attack it until its health is almost fully-depleted. At this point, it'll get tired out and you can rush back to the staircase and pull the lever to finish it off by impaling it to the doors from which it came.
To the Cliffs
Exit through the break in the door it made during its struggle and follow the path until you reach a room containing a coffin. Pull the top off and grab the head of the architect's son. Head back down the stairs to the room housing the Guardian's dead body and to the lightning bolt. Hades gives Kratos his reward, a new power: the Army of Hades. Press and to use it on the soldiers Hades summons. Once that's done, go through the door Hades leaves for you and dive down into the water. Swim through the tunnel to make it back to the Rings of Pandora, and insert the head of the architect's son into the nearby underwater door. The water will be drained out, and you can now move the innermost ring. Align the two rings to make a path to the spiked and bloody halls, and some Legionaries will arrive for battle. Get through them and push the statue to face the other. After the scene, make your way back to the innermost ring and pull the lever to progress to the next stage of the temple: the Cliffs of Madness.