Once you've taken the head of the architect's son, the coffin moves to reveal a new passage. Jump down and follow the path until you reach the dead soldiers. A cutscene follows where Kratos recalls his past. Afterward, keep following the path until you reach a door. Lift it with and make your way back to the Rings of Pandora. A barrier appears to stop you, and enemies soon arrive. You'll have to fight a few waves of Legionaries before proceeding. Once the barrier's gone, head straight to the wall with the skull on it. Press to place the skull of the architect's son and the wall will lift to reveal a large rolling cylinder. Wait for it to pass, then keep running until you find a staircase. Go up the staircase and, once the cylinder comes around again, jump on top of it. Walk the opposite way the stone is rolling until you reach the ladder on the other side. Jump on the ladder when you get to it and a scene follows in which the ladder drops and the cylinder falls through a panel in the floor. After the event, continue to climb up the ladder until you reach the platform with a save point and four chests. After opening the chests and saving, go through the opening and follow the path until you reach the Challenge of Poseidon.
Defeating the Cerberi
Here, Kratos comes face to face with a Cerberus pup that becomes a full-grown, three-headed Cerberus right before his eyes. This battle can be quite a challenge if it isn't done right. As soon as the battle begins, the large Cerberus will attack and Cerberus pups will enter the battle. Focus on the smaller Cerberi first. If you allow these to stick around, they'll only grow into three-headed Cerberi. Ignore the full-grown Cerberus for now and focus all attacks on the small Cerberi for a shorter battle. Use Poseidon's Rage whenever you get the chance. There are some chests around the arena, so the quickest option would be spamming Poseidon's Rage whenever the smaller Cerberi spawn. If you manage to catch the three-headed Cerberus in these attacks, finish it off with and continue directing all attention to the smaller Cerberi until all are defeated and the barrier is removed.
Leave through the path the barrier blocked before the Cerberus boss fight and defeat the enemies that obstruct your path. Eventually, you'll face some more small Cerberi, which should be defeated as fast as possible. Wait for more Cerberi to spawn a couple of times, then continue on the path blocked by archers. Defeat them and the barrier disappears. Enter through the passage and pull the lever with . It descends a few levels to reveal experience chests until it drops to reveal another path. Once you defeat the Legionaries here, keep walking until you get to the ledges in the wall. Climb up the ledges and shimmy to the right until you reach a chest containing a Muse Key. Jump down and repeat the process of climbing up the ledges. This time, shimmy to the left. Continue down the path and you'll find a room housing caged soldiers.
Below the soldiers, you'll find a lever with no handle. Head down the path beyond the soldiers into a room with a handle to the right side of it. Now go back to the lever, place the handle and push it. This will lower a cage containing a soldier. The soldier pleads with Kratos, but Kratos has a quest. Get him on the sloped path to the room where you found the handle. Enemies will spawn to stop you, but simply place the cage in front of the stones to stop it from sliding back down the path. Once you get the cage into the room, push it on the raised square in the middle of the room. Pull the lever and the soldier will be incinerated as a sacrifice. The cage will then be lowered into the ground and the door behind it will rise to reveal a new path. Head down the torched hallway and into the next room. Here, you will find Poseidon's Trident. Walk up to it and press rapidly to pull it out of its place. Kratos now has the ability to breathe underwater.
The Depths of Poseidon's Domain
Jump into the water and a prompt showing swimming controls will pop up. Press to dive underwater and either or to resurface. To the right of the walkway leading to Poseidon's Trident, by the entrance of the room, should be an underwater passage. Dive into it with . Eventually, you'll reach what looks like a breakable wall. Smash through it with and the couple of walls following that. Soon enough, you'll reach the surface. The next room is submerged a bit in water, but not enough to warrant swimming. Defeat the enemies here and lift open the door, then pull the lever behind it. After that's done, go back through the underwater passage you came from and into the trident room. Once you return to the surface, you'll be greeted by a couple of Cerberi. Get rid of them quickly with Medusa's Gaze, then get rid of the smaller enemies. Your next move should be to climb up the blocks of stone pulled out of the wall by the lever you pulled earlier. Here, you'll find another underwater passage. Dive in and swim through the underwater path.
Swim along the underwater path and you'll eventually encounter a Nyad. Kiss the Nyad here with , then proceed along the western tunnel and up onto the surface. Battle the Legionaries here and break past the many urns to get to the chest at the end of the path. Then, go back to where you found the Nyad and take the eastern tunnel. Eventually, you'll enter a tunnel surrounded by rotating spikes. They slowly close in on you, so if you don't take advantage of the underwater dash with , Kratos won't have a very good chance at making it across alive. Keep following the path until you make it to the surface in a room with a lever. The lever is behind a barrier. Defeat all of the enemies in the room and you'll be able to pull the lever with . Your next move should be to go back to where the path that has just been opened by the lever is located. Swim past the new save point and into Poseidon's next trap.
Again, take advantage of to avoid death. Follow the path while occasionally slipping down into the spaces below and waiting for the wall to pass. Eventually, you'll find a passage to replace one of the spaces. Dive down and you'll come across another surface area, this time inhabited by minotaurs. Defeat them, pull the lever, and hurry up the newly-raised platforms. Defeat the enemies and open the chests, then jump back down into the water and pull the lever once more. This time, dive into the water and swim to the last platform. At the bottom, there's a small room that can be entered. Swim into it before time runs out, and the pillar will lead Kratos to a winding passage which leads back to the Rings of Pandora.