Get the Wings of Wrath first: one part of the level will be far easier with them.
In the Heresiarch's Seminary, go to the area closed off by pillars, where the Heresiarch teleported away. Wait a minute, and the pillars will slowly open, allowing access. Stand in front of the centermost section of wall with the candles on either side and wait. When the pillars close, a lift you're standing on will lower. At the bottom is the portal to Deathwind Chapel.
Deathwind Chapel[edit]
You begin in a small room with two stone doors, a wooden door and a portal that returns to Heresiarch's Seminary. The level consists of three areas you complete successively, each with assorted puzzles and bonus areas.
The reward for this bonus level is a Krater of Might, although you can technically harvest it from the Heresiarch's Seminary from the monster spawner.
First area[edit]
Enter the wooden door. After clearing out the enemies, pull the switch at the far end of the room. A stairway will rise on your left.
Upper half[edit]
Up the stairs is an overlook above a large lava-filled room. At either end of the room, to the left and right, are additional overlooking rooms you can see through windows, plus a corridor connects them beyond the far side of the lava room. Another lower room is down at lava level, opposite from the overlook you entered from.
It's this room you'll want the Wings of Wrath for, because without them you'll have to run around on the lava. From the first overlook, look for a doorway down at lava level on the far right of the room. Fly (or run if you have to) over to the doorway, and follow the passage up to the right-side overlook. You won't see the connecting corridor; a hidden door to it will open from the other side. Find and pull the switch in here, then go back out to the lava and to the lower room. If you went here earlier, you'll have seen the room has front and back halves, and a door to the back half was closed. The switch opened the door. In the back half, take the lift you find, which goes up to the connecting corridor. You can get to the right-side overlook now. Find and pull the switch in the left-side overlook, and take a look around and collect the items. Go back to the lower room. The switch you just pulled opened the metal column in the back half. Hit the switch that's inside the column. This opens a door in that short passage down at lava level outside the left-side overlook, leading back to the first room.
Lower half[edit]
Back in the first room, another stairway has lowered on the other side of the room. Down here is a big, tall room made up of a deep pit, with a walkway leading down and along the left wall to a lower overlook on the opposite side. Watch out for a gang of Slaughtaurs down on the overlook. Go to the overlook, jump out to the column in the pit, pick up the Platinum Helmet, and wait. A series of steps is rapidly rising up out of the pit, connecting to the walkway you entered from.
Before continuing, there's a secret area nearby. Go back to the overlook, and look out on the side opposite the walkway you first came here from. There's a hidden compartment up on the outer wall, at the same level as the overlook, covered by a one-way window. Run and jump off the overlook and into the wall, and you'll land inside it. (You can see it on the automap, if you're uncertain.)
Go up the steps that formed in the pit, and go left and follow the walkway up and through a narrow passage to where it dead-ends. Pull the switch here: a door ahead opens, connecting to the first room. (if you went here earlier, the switch would have been covered up.) Go back to the room where you entered the level. One of the stone doors is now open.
Second area[edit]
In here is a long, fairly narrow room. There's a huge horde of Centaurs and Slaughtaurs in here, most of them inside compartments on either side that will open shortly. When you get to those eight square columns halfway down the room, they'll begin to spin, slowly at first and then more rapidly. They'll also begin moving side to side in an undulating pattern. You'll need to go between the two rows to get across the room. If you get caught among them you'll be crushed. But if you watch the side-to-side motion, you'll see how you can follow the pattern to get past them safely. You can also go across right away before they begin moving together.
At the far end of the room, hit the switch you find. Now's when all the compartments will open, releasing the remaining enemies, plus the exit will close off. You'll need to get past the spinning columns again, but the side-to-side pattern is going the wrong way to follow back out. Wait until the closest pair of columns are just far enough apart to get through, then run through them to make it past the last pair before those ones are too close together. But, like before, you can quickly hit the switch and get back across the room before they begin moving side to side. You'll have the enemies on the first side of the room to deal with still. Discs of Repulsion, and a row of Flechette gas clouds/time-bombs if you're playing the Cleric or Mage, will work especially well here. After a few of them are killed, the exit will be unblocked.
The switch opened the remaining stone door in the starting room. Before continuing, you might want to look in the opened compartments. There's a lot of ammo and even some weapons available.
Third area[edit]
Past the second stone door is a long, narrow room with rows of windows on either side looking into side rooms crammed with Centaurs. At the far end is a closed stone door. When you go to the other end, the windows will convert into doorways, releasing the enemies. It may be easiest to go back out to the starting room and fight them through the doorway. If you're playing the Cleric, a trail of flechette gas clouds down the middle of the room, placed as you leave, will make short work of the Centaurs. On the far sides of the two side rooms are doorways to two additional side rooms, each of which has a teleporter. (This whole area is made up of a total of five rooms side-by-side.)
Take one of the teleporters: it goes to a small room looking through a narrow window down into a high courtyard with a small pool and a statue. You'll see two closed stone doors on either side of the courtyard. Look for and pull the switch nearby, and one of the doors will open. The teleporter in this room goes back to near the teleporter that led here. Go to and take the other teleporter in the opposite side room: this goes to an identical room looking down on the courtyard on the opposite side. Pull the switch here to open the other stone door. Go back to the center room, and you'll see the stone door in here is actually one of the two in the courtyard. There are lots of items here, plus the other stone door goes to a compartment with a Krater of Might.
Once you've cleared out the three areas, your tasks here are completed.