Climb the wall to reach the tunnel above then open the gate. Scale the wall to the right then make your way to the upper ledge. Continue forth until you find a lever and a room with red orb chests. Open the chests then interact with the lever. The caged enemies you passed by will be released so take them out.
After eliminating the enemies, enter the cage where the first set of enemies were released earlier and inspect the floor in the left corner to find the first artifact in the game. (Prisoner's Oath Stone)
Head to the right next and pull the large chain to open the gate. Recover your health using the green chest in front of the gate then continue through the corridor. Walk until the prison cell block you're in gets detached by Aegaeon's massive hands. Continue fighting off groups of enemies that will be shaken off from their cells. After taking them out, head to the edge of the block and grapple on the glowing green point to reach the next section.