The Walkthrough - Part 5

The Plane Truth
If you have trouble finding the underwater tunnel (its dark), switch to the Spectral Plane to get a better view from the base of the submerged pillars. It is on the eastern side of the lower chamber. A Material Plane Portal is there too, so you can switch back.

When you surface, you must deal with three Shades, one of which fires projectiles from a distance. Either turn to the right and climb partway up the ramp to take care of him first, or keep moving and get him at the end.
After you kill the Shades, head up the ramp along the east wall. Jump across a gap at the top of the ramp, right before the glowing crystal. Continue across the bridge as it turns north.

Tip
Hold R1 while walking across narrow bridges and ledges to avoid falling off. If you move too slowly, nearby undead Shades will peg you with projectiles.

A Shade lurks near the crystal. Dispatch it, and then continue east toward another crystal and Shade. There's a cliff to the northwest; peek over it (hold R2) to see a Shade waiting below.
Jump down and deal with the Shade. Don't climb up the cliff. Instead, head north to the dark pool of water.

Underwater Spelunking
Jump in and swim past the large, submerged statue toward another underwater tunnel entrance in the north, also lit by a pair of glowing crystals. You can warp to the Spectral Plane if you have trouble orienting yourself, but you must be in the Material Plane to continue to the next area. The Material Plane Portal is on the opposite side of the statue from the tunnel entrance.
Navigating this tunnel is difficult because it's quite dark. From the two crystals at the entrance, head toward the first crystal. There is a second crystal farther ahead. At the second crystal, there is a third, and from the third, there is a fourth.
Beyond the fourth crystal, there are four circles arranged in a triangular shape. This is an inactive Air Plinth. From here, head up and northeast. There are a number of crystals above you; continue to the top of the cave, and follow the uppermost tunnel. Continue east, and then follow it as it goes directly up. Jump out of the pool.