You need to jump into the lava and run to the opening on the right (make sure you see where it is before you jump! It's not fun walking around on lava looking for something you can't find). There's a narrow staircase here that leads into a dark room with Dark Bishops. Activate the switch in this room. This switch activates the elevator in the next place you go to.
Now, go back down to the lava, and run across to the opening. Go inside the room with a pillar. Step on the elevator behind the pillar, which takes you to two rooms where Dark Bishops were shooting at you earlier. There's a door in the room on the right which takes you back to the room where you activated the elevator switch. Go to the room on the left and down the dark passage to another room with Dark Bishops.
Activate the switch in this room. This opens something in the pillar in the elevator room. Go back there and examine the pillar. Activate the skull switch. Now a wall has opened on another side of the lava, which leads back to the first room in the Lava area. Run across the lava into the room. Now stairs have lowered on the other side of the room.
Go down the stairs to a room with a large pit. Follow the ledge on the left carefully. There's a pillar in the middle of the pit with a platinum helmet on it. Go to the ledge nearest to the pillar and jump on the pillar. Stairs will rise and join this pillar to the entrance of this room. Go back to the entrance, and now follow the ledge in the other direction. This leads to a switch which opens a wall back to the first room and also opens the door to the Centaur Room.
After you bash the guts out of the monsters, go inside the room. There are two teleports here, one on each of the dark sides of the room. Each teleport takes you to a small area which overlooks a courtyard with a statue in the middle. Pull the switches in both of these areas. This will open the door at the far end of the room, which leads to the courtyard with the statue. There are lots of goodies here, with a Krater of Might in an opening at the far end of the room.
That's all there is to this level. Collect all the goodies and ammo here and step into the portal that leads back to the Heresiarch's Seminary. Now you're probably more equipped for the next episode! Step into the portal that leads to the next episode, the Castle of Grief.