The level itself is a killing floor connected with an underground library, dungeon, balcony, hypostyle hall, and throne room.
You’ll start on a platform very similar to the one you just left. Pulling the bullhead switch, you’ll descend into the central area of Gibbet, where several Afrits will greet you. The circular area has a pool in the center with a Stalker Boss in it (or a Stalker if you're the Fighter). To the left is a portal back to Castle of Grief. There are Flechettes around the perimeter.
You can explore the adjacent rooms and corridors to an extent. But the first objective is to solve the Yorick's Skull puzzle. First of all, you'll need the Liber Oscura and Daemon Codex, the two puzzle items from Forsaken Outpost. If not, you’ll need to take the portal back to Castle of Grief and go there. Assuming you have them, you’ll exit the wide southern opening of the circle, and go straight ahead down the southern corridor. The second bookcase on the left is the door to the library. It can damage you while it rotates, so move through swiftly. Once inside, you’ll fight numerous Ettins, some of them behind the stacks. Once you’ve cleared them, head to the eastern slanted wall. The easternmost bookshelf of the center row is the key - it has a unique design with a skullface on top, like the door. There are two openings in the row of books, where you’ll place the Liber Oscura and Daemon Codex. Once you’ve done this, the room darkens and several bookshelves lower to unleash Dark Bishops from the ceiling. In the middle of the bookcases, you’ll now find Yorick's Skull.
With this, head back to the circular corridor and go to the eastern entrance on your right. You’ll enter a large pillared hall with many Dark Bishops. The stained glass windows mostly conceal Quartz Flasks. The southernmost window has a skullface switch guarded by a Slaughtaur (and the Mage’s third weapon). The switch will raise a bridge across the crevice to the headless Yorick's statue. The northernmost window has a Slaughtaur and the Fighters third weapon, and the northeast window has a Combined Mana container. Place Yoricks Skull on the statue, and be careful. After a short time, the room will quake and the floor will fall away, except the path demarcated by the torches. It is important to have activated the skullface switch, otherwise there will be a large gap between the statue and the path that is not easily jumped.
From here, return to the central circular room. The pool in the middle will have lowered to reveal a teleport switch on the pillar. This brings you onward to Effluvium.
Returning from Effluvium, you’ll find yourself back in the central pool. You should go back down to the southern corridor, but instead of the library, you’ll be going through the first bookcase door on the right. You have to be careful of it the same way, and you’ll pass through the left side instead. Inside, there is a bedroom with Centaurs roaming and a Slaughtaur in a barred area. When the Slaughtaur is defeated, a passageway will open on the left. Another left turn brings you to a storage area with more Centaurs. There is a Quartz Flask and several Fléchettes around the barrels, but beware. The darkened portion of the wall indicates a small death trap pit near the southeast corner. You’ll come back to this area later. You can enter the Slaughtaur’s barred area to pick up a Porkelator artifact. Back in the bedroom, there are several things of interest. First of all, don’t be lured by the Crystal Vial on the four-poster bed - it is a crusher trap. The pots in the northeast corner contain the Fighter’s third weapon. Activate the nearby bookcase, and you’ll descend to a basement with Ettins, Centaurs, and Crystal Vials for your trouble. Dark Bishops will see you through a window at the southwestern end, and you can take them out early with projectiles. The bullhead switch will take you back up. The raised wooden area has Dragonskin_Bracers and a bullhead switch on the wall. Pull it, and stay right near it. The wooden floor will disappear and create a pitfall. You can jump toward the side ledges at an angle to safely get out. This switch will also have opened a necessary passage in the dungeon area. At the far western end of the bedroom, there is a Dungeon Key door. As you open it, the doors swing outward. Inside, there is a dungeon full of cages with corpses inside, and Ettins. There will be an opening in the northern wall adjacent to the entrance, provided you pulled the previous switch. If you didn’t take them out earlier, there will be a few Dark Bishops to deal with. The westernmost end to your left has another bullhead switch. Pull it, and you’ll hear an opening sound. You’ll have to go back into the bedroom, and into the southern storage area. Now, there will be a narrow staircase descending along the southern wall across from you. As you reach the bottom, be wary of the small pitfall on the left. Turning right will bring you to a room with a portal to Dungeons, where you'll continue your quest.
In the southeast corner of the portal room, there is a narrow staircase leading upwards. At the dead end, shoot the wall in front of you and it will open to the library. This provides a shortcut between the Dungeons portal and the rest of the level if needed.
There is a script on this map that requires a very specific condition to activate that the player can accidentally render impossible to meet. The script must activate to allow further progress in the game.
In the room behind the Axe Door is a throne that four Chaos Serpents will teleport on to. The aforementioned script checks every few seconds that exactly three remain and lowers walls when the condition is met. However, it is possible for the player to reduce their number below three in those few seconds. In this case the script will never activate and the player will become permanently stuck.
If this should happen, use the "casper" cheat code and go through the walls on either side of the room, killing the Dark Bishops inside both hidden rooms. This will trigger the back wall to lower, revealing the second Heresiarch's lair. The Heresiarch may not be responsive, but attacking him will wake him up, and the game can proceed from there.
This bug was fixed in the version of the script included in the Version 1.1 WADfile.