The Gibbet has several rooms and places:
Note: there's a pillar in the chasm with many Repulsion Discs. You can jump to it, but only if you're very expert. I suggest that you solve puzzles here first, and then come back, because a magic bridge will appear after you solve the puzzle.
Enter the doors, which lead into a courtyard with a statue in the middle. There are two small rooms on the left and on the right. There's nothing in them yet, so go to the far end of the courtyard where there's a dark corridor. The big doors in front require the Rusted Key, which you'll get later. If you go right on the corridor, it will lead to the ledge where the Chaos Serpent is. There's a platinum helmet here. If you go left on the corridor, it will fork into two. The left turn goes to the other ledge with the Chaos Serpent, where there's a quartz flask. Follow the right turn, which leads into a small dark room with a switch.
Pull this switch. This opens a new room at the corner of the passage where you came in. Go inside the room and kill the ettins there. There's a book lying on a pedestal in the middle. Wait! Don't forget to pull out your weapons before you grab the Liber Oscura. This mystic book has many jealous guardians! It's one of the items you need in the Gibbet.
The new passage leads into a dark room with a shallow chasm in the middle, and some Dark Bishops. Jump from pillar to pillar to get to the ledge at the other side of the room where there's a switch and a Porkalator. Unlike many other places in this game, don't worry if you fall into the chasm. It's quite shallow, and there are some high steps that let you climb back up (the steps are high, so you need to jump in some parts). Pull the switch on the ledge.
If you're observant enough, you'll notice a grey ball lying on another ledge in this room. This is a sleeping Afrit... unfortunately you can't kill it yet. It's invulnerable in this "ball" mode.
After pulling the switch, another area has opened in this dark room. There's some ammo there and a secret window which looks into the room behind the door that needs the Rusted Key (it's a secret window because you can't see it from the other side). There's also a door here that will open much later.
Go back to the courtyard now, and open the big doors with the Rusted Key. Watch out, though. There are many unpleasant monsters behind the doors.
The doors lead to a big room with many crystal vials and quartz flasks. There's also lots of ammo at the far corners. At the far end of the room, there's rusty pillar. You need to open it by pulling four switches. The four switches are in four small rooms at the sides of this room. You must kill the monsters inside here before the rooms open. Once they open, go inside and pull the four switches. The rusty pillar will split into two, move to the side, and the wall behind it will slowly lower. There's a book lying on the floor beyond. This is the Daemon Codex, which you need in the Gibbet.
Now that you have both the Liber Oscura and the Daemon Codex, go back to the portal that leads to the Castle of Grief. Along the way, you might want to grab the Discs of Repulsion on the pillar in the deep chasm. Note that you must jump onto the blue magic bridge, and jump back; it's not really a bridge but just some kind of 'stepping stone' that helps you get to the pillar. Be prepared for some jealous monsters while you're grabbing the Discs, though.
Go back to the Castle of Grief, and from there, go to the Gibbet: you're now ready to solve the next puzzle. (Or, finish solving the puzzle of the Clock first in order to get to the Gibbet, if you haven't done so already.)
The library has many bookshelves. For some reason, there are several ettins here (I don't know for what. In my opinion, they are really stupid blockheads; so dumb even though they've two heads. So it's surprising to find them in a library!!). Look for the bookshelf that has a demon mask at the top. (Hint: this bookshelf is the last one in its row, so it's facing the wall.) There are two empty slots between the books. You guessed it! This is where the Liber Oscura and the Daemon Codex belongs. If you look carefully, the letters on some of the books on the shelf spell out K-R-X. Notice that the Daemon Codex has the letter 'O' and the Liber Oscura has the letter 'A'. Pull them out of your pocket, and slide them into the empty slots on the shelf, so that the letters spell out K-O-R-A-X! The lights go out. Whip out your weapons and prepare for another battle...
Several of the bookshelves have lowered into the floor. One of them has a skull with horns on it. Pick up this skull. This is Yorick's Skull. Just like the two books, this skull also belongs somewhere... can you think of where?
Go to the Statue Room now. If you haven't been here yet, go and smash all the stained glass walls around here! Behind one of the stained glass walls there's a switch. Pull this switch. Now the broken bridge that leads to the statue is repaired. Walk up to the statue. Notice anything unusual?
OK, you guessed it. Return Yorick his skull. Watch your step now! Remember I warned you about tectonic instability here...? When things have calmed down, walk back on the path and leave the room. The tremor has created a leak somewhere. Go to the Central Room now. The pool here is leaking through some leak near the black pillar. Go down to the pillar and you'll see an eight-pointed star. Flip this symbol, and oops...! You've been sucked into:
You start in some kind of drain, which flows ahead into a pool of muck in the room at the far end. Before you go there, you need to look for a switch in one of the small ledges on the side of the drain. If you're observant, you'll notice doors and walls in the other ledges that look like they might lead somewhere interesting. They do; however, you can't open these places yet, at this point, so just find the switch and activate it.
The switch opens a portal back to the Gibbet in one of the other ledges, and also lowers another drain on the other side of the room with the muck pool (which lets in some nasty shooting Stalkers as well). Jump into the muck pool, and go to the end of this other drain. There's a ledge on the right, which leads into another room filled with muck. There are windows in this room that look out to the outside world. The door at the end of the room lets you out to breath some fresh air. :) If you look carefully enough, one of the windows has something on its sill on the outside. This is the Dungeon Key. Go out the door and turn left. Walk over to where the windows are and grab the Dungeon Key.
This is about all you can do in the Effluvium at this point. Go back and step into the portal that the switch opened earlier, to get back to the Gibbet.
Kill the Slaughtaur in the cage, and a wall beside the cage opens. This leads inside the cage itself, as well as into a room with many barrels. There's a secret passage in this barrel room that isn't open yet, so go back to the room outside the cage.
The bookshelf in the corner lowers a platform underneath you if you activate it. This leads to a short passage with some monsters and a window beyond your reach that looks into a room with Dark Bishops. That room is behind the door that needs the Dungeon Key. To get out of this passage, pull the switch near the platform, which lowers it and takes you back to the first room.
You need to pull the switch at the end of the wooden ledge. Watch out! As soon as you pull the switch, the wooden ledge collapses into a really deep pit! NO! you don't try to run back after you pull the switch. The ledge collapses too fast for that. Rather, you must stand on the tiny piece of wood that's left in front of the switch! Then, slowly turn around, and jump as hard as you can to get back on solid floor! (Hint: holding down the Jump key longer will help here... also, try to run forward at the same time.) This switch opens the room with the Dark Bishops that you saw earlier.
Now, open the door with the Dungeon Key. Go inside the place that looks like a torture chamber, and turn right. The wall here should have opened (when you pulled that switch), and the room with the Dark Bishops is beyond. Go inside that room and pull the switch there. This lowers stairs in the room with barrels. Go to the room with the barrels now, which is behind the secret passage beside the cage in the first room.
Go down the stairs at the far end of the room. Be careful here! At the bottom of the stairs, turn RIGHT. There's a very deep hole on the left, so don't ever back up into it! There's a portal in the room here, which leads to the Dungeons. Step into the portal now.
Go to the west side of the room now, where there are stairs leading into a really dark torture chamber. Go inside the chamber (watch out for monsters!) and go the west end. There's a switch here you must activate. This switch opens the other switch on the pillar in the large room with the lava, and also opens the bars blocking the south passage.
Go back to the large room and enter the south passage that is now accessible. This leads into a round area with seven cages. In the centre of the cages there's a wooden pillar. A switch is hidden in this pillar, but it won't open just yet. Go back out to the lava room, and press the switch on the wooden pillar. Each time you press this switch, one of the seven cages open. You can either open them all by pressing the switch seven times all at once, or, (recommended) press it once at a time, go to the opened cage and kill the slaughtaur first, before you come back to press the switch the next time. After all seven cages are opened, another switch appears in the pillar in the centre of the seven cages. Go and press this switch. This opens two new passages on the west side of the lava room.
These new passages lead to the walkway above the really dark torture chamber. You can jump on the pillars with the goodies from here which you couldn't reach before. There's also a couple of secret doors on the pillars that lead to the goodies on the high ledge that you can see from the lava room.
Go up the stairs on the southern passage, and take the first left turn. This leads to two rooms with ettins. There's a porkalator in the farthest room. Come back out and continue along the passage. The second left turn leads to two cages with some nasties inside. The switch on the wall is worth mentioning here: if you pull it once, it opens the door to one of the cages. The second time you pull it, it opens the other cage. The third time you pull it, (notice the red eyes this time) it closes both cages.
Continue along the passage, which turns a corner at this point. Take the left turn, and you'll get to a room with small cages and mutilated bodies in them. There's ammo at the far end of the room. Go back to the passage and continue. The next left turn leads to four rooms. The farthest room, on the northwest, (the one with many chains) has a secret door on the west wall, which leads to a long corridor that takes you to the room with the slaughtaurs at the end of the passage outside. There's plated armor here. Activate the switch, and the windows will open so that you can get out without having to retrace your steps.
Go east now, along the northern ledge. A new passage has opened on the left, which has a different wall texture. Go down the passage to a room with a gang of Chaos Serpents and several cages at the sides. There's another passage on the right that leads to another room with Chaos Serpents and cages at the sides. Stairs from one of those cages lead to the room on the east of the lava room. If you go down the stairs at the far end of that room, you'll get back to the lava room.
AAaaaaarrrrrrggghhhh... !!!!!! You've fallen into a pit! No! don't reload your game! Just hang on while you tumble into the bottomless pit...
If you can't wait to solve the puzzle, go there now and skip the following digressions.
At the end of the journey, you'll come to a very high room filled with muck. On the opposite wall, the muck is draining out from the room through a drainhole. There's a winch on the left. Turn this winch now, and the hole closes. The muck will start to fill up the room... until the water level (or muck level, to be precise!) reaches two overflow-drainholes at the top of the room. You can now reach the ledge on the right side of the room where there's a Mystic Urn and another winch. Grab the Urn, and turn the winch. Quickly jump back into the muck; the winch has re-opened the drainhole at the bottom of the room, and the water level (muck level) is dropping back to its normal level. If you don't jump back now, the water level will be so low that you'll get hurt jumping back in.
OK, that's enough disruption of sewer operation. :) Go back to the room with ettins and go to the place where the portal to the Gibbet is. A wall has opened on the ledge opposite the portal. There's a labyrinth of muck rivers beyond. Go inside there now. If you look carefully, you'll notice a Porkalator on the other side of a drain hole. You'll get there shortly... Now, go down the drain the leads from this room. It branches into three. Don't worry; it's not as confusing as it may seem.
On the left, those walls are the walls of a room with a torch in it. I call this the Torch Room, because there's a torch inside to help you see more clearly in these dark drains. The entrance is on the other side. If you look up in that room, you'll see that you're looking up through a sewer grating.
The walls on the right are walls of another room, which I call the Dragonskin room. It's not opened yet, so now, follow the drain on the right, and keep to the right wall. You'll see the walls of a third room on your left. Look along the wall on your right carefully. Open the wall where there's a rusty handle. There are two such walls; one leads into a room with a switch and a window looking out of the sewer, and the other leads to a platform that actually takes you outside. Activate the switch, and the walls of the third room lowers. There might be some monsters in that room. Now, the Dragonskin Room is open. There's an elevator there that takes you up on a ledge with Dragonskin Bracers.
Go to the platform that takes you outside to a small dark area with some trees. Look for another sewer hole in this area, and jump down into it. Go right. There, you got the Porkalator you saw earlier on. To get back, climb on the rusty platform which takes you back out to the area with trees. Jump back into the hole where the three rooms are. Now go back to where the portal to the Gibbet is.
There's a winch between the two tanks on the north side. Turn it now, and the muck will drain way from the two tanks. Jump into the northeast one. There's a falcon shield in a small room to the north. The pillar on the south side of the tank is an elevator; activate it to get back out. Now jump into the northwest tank. There's a portal in the room beyond that takes you to back to the Castle of Grief. You'll come out in a hidden room at one end of the Castle's moat. You can raise the wall to get out onto the moat and into the Castle. For now, step back into the portal to get back to the Effluvium.
The portal will take you to the west side of the room with the four tanks. Jump onto the southwest tank now. There's also a winch between this tank and the southeast tank. Turn the winch to drain out both tanks. The southeast tank leads to a small chamber that sometimes has an item. Again, the pillar in the tank is an elevator to take you back out. Go to the southwest tank. There's a wall with a handle in the room beyond. Open this wall, and you'll see another room behind the muck drain, with a Brown Serpent. Watch your step here! There's a pit right in front of you! You must jump over it to get to the room with the Brown Serpent.
This room has a waterfall (muck falls, to be precise). There's also a rusty pillar which is an elevator. Activate it to lower it, and it will take you up to a door which opens into one of the ledges beside the drain with the portal to the Gibbet. Don't go back to the Gibbet now; you haven't solved the puzzle yet. Go back to the room with the muck falls. Climb on the ledge where the Brown Serpent was. Notice the wall here that has a handle on it. Pull it open to reveal a portal. Step inside.
Step back into the portal that leads back to the Effluvium, and take the portal in the Effluvium that goes to the Gibbet. Now you're ready to solve the final puzzle in this episode!
On the right side of the room there's a cage with some monsters inside and a pillar. If you've solved all the puzzles so far, the southeast corner of this cage will be open. Go inside and the pillar lowers, revealing the long-sought Axe Key. Grab the Axe Key, and go to the door of the Heresiarch's Lair. Now you're ready to face the Heresiarch! Open the Heresiarch's Lair with the Axe Key. Note that the door will close after a short time, and it cannot be opened from the inside. You must make sure you have enough health potions and ammo to face the Heresiarch plus lots of monsters before you enter this door. Once you go in, there's no return until you kill the Heresiarch (unless you have the Chaos Device, of course).
Note: Some people, including myself, have experienced problems in this room, where some script malfunctions and the Heresiarch doesn't appear. So I advise you to save your game before you enter the Heresiarch's Lair, in case something goes wrong and you're stuck. I'm not sure exactly what causes this problem; many people have mailed me about this, but everyone seems to have a slightly different version of the problem. For my case, I must jump on top of the throne where the Icon of the Defender is, as soon as I enter the room. Just grabbing the Icon without jumping on the throne or jumping on it after the Afrits appear seems to be the cause of my problem. Other people have mailed me different solutions, including avoiding the god-mode cheat, opening the Axe door more than once, etc.. If anyone knows the real answer to this, please tell me!
Enter the Heresiarch's Lair and kill the monsters there. Now, go up to the throne and jump on top of it when you grab the Icon of the Defender (see the note above). This opens pillars at the sides of the room with lots of Afrits. Kill all the Afrits, and Chaos Serpents will teleport into the room around the throne.
Now, here is where the malfunction I mentioned above may occur. If after you kill the Chaos Serpents and waited a long time, and the Dark Bishops haven't appeared yet, something's wrong. You should reload your saved game and try again. You must kill all the Chaos Serpents as well as all the Dark Bishops.
Then, a wall behind the throne opens, with a big star of lava on the floor and lots of quartz flasks lying around. The Heresiarch is standing on top of the star, prepared to beat the life out of you! You might want to review the tips on fighting the Heresiarch which I gave in Episode 3.
After you kill the Heresiarch, the portal behind the lava star opens. This portal leads to episode 5, Necropolis. Also, the door to this room is opened momentarily when the Heresiarch dies. If you want to get out of this room to explore other parts of this episode, you should run to the door as soon as the Heresiarch dies. Once the door closes again, there's absolutely no way out of this room except with the Chaos Device.