Retrace your steps to the spiral staircase (hit the button near the chasm to extend the bridge). Go down and take the 1st door on the right. Come to a courtyard.

Climb up the rampart to a room. Take the secret passage to the lift. Take it up and go through the door to the Dark Mage's Laboratory.

Crawl to the area behind the chair and place the gold sphere between the two electric prongs.

Step on the teleporter between the 2 flickering metal balls back in the lab, taking you to an outdoor arena with a golem on the left. Defeat him to open the door at the top of the stairs.

Go through and follow the hall to an outside room with a statue, ring of flying, and a book high up on a shelf. Smash the front left corner of the statue's base to topple it. Grab the Ring of Flying to fly up and grab the Mage's Tome.

Turn around and fly through the red teleporter mural, taking you back to the spiral staircase.

Take the stairs down through the grate and return to the Underhalls.

Go back down the steps, turn left and continue to follow the tube. Drop down into the water and turn the wheel on the wall to get the Icon of the Defender. Swim up through the hole on the right (closest to where you found the Icon) to enter the torture chamber.

Notice the two doors. Open the one on the left and walk up to the cell's right-hand wall.

When it crumbles, notice where "help" is written on the wall of the adjoining cell. Shoot the wall to get a hidden Krater of Might.

Climb up the stairs to leave the torture chamber and return to the room with the pedistal in the middle. Place the Mage's Tome on the pedistal, opening the mural. Enter the double doors to Eidolon's Ordeal.