Two of the five planet gems you need are in this level. You enter the level in a small cave, in front of the portal that returns to Heresiarch's Seminary. Outside the cave is a fair-sized outdoor area divided by several high walls. Follow the route outside. It crosses over a muck stream coming from a fall overhead to your left. Watch out for Stalkers.
Beyond, the route continues to an open area with several high overlooking areas on either side. Beyond the stone walls is a long narrow area, near either end of which is a stairway leading down and away. Both stairways lead to a big square yard, with a narrow ledge around three sides with items (reachable from the tops of the stairways), and a network of sunken walkways.
Lower rooms[edit]
After clearing out the enemies, go down to the walkways. The left and right ones don't go anywhere at present. Go down the one ahead, down a stairway to a small room under the outer wall. Find and pull the switch in this room, and watch what happens. A small lift at the far end of the room momentarily lowers. Pull it again, and this time ride the lift down, but stay on the lift the first time and watch carefully what happens. You'll see a large square area the lift leads down to, but the floor of the area also lowers and rises in time with the lift. If you're standing in this area when the lift goes back up you'll be crushed. Here's how this works: along the outside of the area with the crushing floor are a total of eight small rooms including the one you're now in, each with a lift and a switch that lowers the crushing floor and all of the lifts. Look at the automap and you'll see how it's arranged. What you'll need to do is work your way around to the room on the opposite side of the area from where you're at now. It's safest to take it room by room. The rooms each have an Ettin or two that might get in the way, so look for them. (It's a good idea to go to every room as you complete the area, because all have items.)
In the room opposite from the one you entered from, you'll find a second switch. This opens up a room in that central area the crushing floor surrounds. it also releases a bunch of Dark Bishops: try repeatedly lowering the crushing floor and get them to fly into the area. The floor will catch and crush any that do. Next, go to the opened central room, where you'll find two switches. One lowers the lifts and floor. Find and pull the second one, then go to any remaining rooms if you want, and go back outside.
High central area and Emerald Planet[edit]
Return to the upper area with the high walls. The switch in the central room you came from has lowered some of the overlooking areas, and you can now jump up onto the lowest of these. Work your way to the top of this area, and you'll find a muck pool that's the source of the stream you crossed earlier. Watch for Stalkers here too. At the far end of the pool is an Emerald Planet.
Magical tunnels and Sapphire Planet[edit]
After you pick it up, go back down to the square yard. The two sunken walkways to either side now have stairways leading down under the outer wall in the same manner as the middle one, plus a side room in the yard has opened, with enemies and items including a Porkalator. If you step into the room, you'll be ambushed by Slaughtaurs from two more compartments that open on either side. There are more items inside.
Go down both of the stairways. Both go to magical tunnels like the one back in Heresiarch's Seminary, and connect to those two ledges along the outer edge of the upper area, where the Slaughtaurs are/were. On one ledge is a Sapphire Planet; the other has armor.
After you've collected both planet gems and seen everything you want, your tasks are finished here.