Your objective in the Tomb of Souls is to make your way to Voraatus and then "bargain" with him to get Vartugg's eye back. You'll have to face some tough creatures in the tomb, including many demons, so if you haven't purchased any crystal wards of fire from the Temple of the Empyrean in Arindale, you might want to do so before going inside.
You'll also find several high level chests inside the tomb. Remember that chest contents are semi-random, so if you save your game before opening one, you can load your game until the chest gives you something that you need. But be wary; many chests here release creatures (typically bats and succubi) when they're opened.
1 - Locked Door
You'll need to acquire a gold key (#2) to unlock the door.
2 - Sarcophagus
When you open up the sarcophagus, a demonic guardian will pop out. The guardian will drop a gold key when it dies. You can use the key to unlock the door at #1.
3 - Secret Passage
You must press one of the nearby buttons (#4) to open up the secret passage.
4 - Buttons
5 - Secret Door
The secret door will open once you've moved close enough to the next room. A bunch of mummies and skeletons will come out of the secret room and attack you.
6 - Enkhar of Evil
"Enkhar" must come from the same dictionary that contains "chunneler." Anyway, the enkhar is some sort of lock, and when you place the eye of evil (#10) in it, the door to the west (#7) will open.
7 - Locked Door
You must place the eye of evil (#10) in the enkhar of evil (#6) to open the door here.
8 - Sepulcher
When you approach the sepulcher, several succubi and bats will jump out of it and attack you.
9 - Sepulchers
The sepulchers can't be opened. Enemies will pop out of them when you take the eye of evil (#10).
10 - Eye of Evil
You'll need the eye to open up the door at #7.
11 - Locked Door
To open this door, you'll need to have the demon key from the Naga Temple.
In the room beyond, you'll discover the Fiend of Ghazbu. The fiend is a big, bad demon. It will cast two different fire spells, plus some other spell. If you have a decent level of celestial magic, then you can use crystal wards of fire to block most of the fire damage, but the demon will still be tough to kill, just because its endless stream of spells will continually knock you back and knock you down.
However, there's a trick for the demon. If you retreat back the way you came, the demon will get stuck at the sepulcher (#8). Then you can stand in the hallway to the north and zap it with ranged attacks. Strangely, not only will the demon not move beyond the sepulcher, it will also largely forget that it can cast spells, and so you'll be able to attack it in peace, except for the odd random encounter that will pop up during the fight.
The demon won't drop anything when it dies, but if you can kill it, you'll receive well over a million experience.
Note: The fiend was changed in the Collector's Edition. It now summons demons to help it, and it was given an insanely powerful magic attack. This makes it much tougher to kill (the summoned demons will cause it to move around, taking it out of the "sweet spot" behind the sepulcher). If you can kill it at all, it will probably take lots of loading and saving.
12 - Treasure Chest
The chest is underwater. Inside the chest you'll find a skull key. You can use the key to unlock the door in the fire room (#13).
13 - Fire Room
Two statues will blast fire into the room every time you enter. That sounds bad, but running into this room is an excellent way to get rid of any pesky bats that might be following you.
The door on the western side of the room starts out locked. The lock isn't right next to the door, but it's nearby, in the northeastern corner of the room between the two fire statues. To unlock the door, you'll need the skull key (#12).
14 - Watcher Challenge
In order to meet with Voraatus, you'll have to pass a challenge in this room. To take the challenge, talk to the Watcher on the western wall and agree to its terms. You'll have to defeat four spirit fiends (demons) without healing yourself. No doubt there are a lot of strategies that work here, but we just used a crystal ward of fire before talking to the Watcher, and then we whacked away at the spirit fiends until they were dead. Summoning is probably effective here as well.
Once you've defeated the fiends, talk to the Watcher again, and it will open the secret door to the south (#15).
15 - Secret Door
16 - Button
Pressing this button will raise the collapsed bridge (Exit B), allowing you to cross it and bargain with Voraatus (#17).
17 - Tomb of Voraatus
When you open the tomb, Voraatus will walk out. He's not necessarily an enemy, so don't start swinging your weapon. Instead, talk to him to learn what it'll take to get Vartugg's eye back. Voraatus will tell you that you'll have to play a game that involves guessing the first five tortures that Vartugg will experience when he enters Voraatus' realm.
The game is essentially Mastermind, if you've ever played that. Each time you make a guess, Voraatus will tell you how many tortures you've gotten correct, and he'll take away attribute points for each one you've gotten wrong. The order of the tortures is random, so you'll have to figure it out on your own. However, realize that you can select a torture multiple times per guess. For example, if you guess "fire and ice" three times and "disease" twice, then if Voraatus tells you that you've gotten two or zero tortures correct, you'll have learned a lot about where those tortures go.
Of course, even if you're good at the game, you'll still lose quite a few attribute points. Plus, after you've completed the game, you'll have to fight off a bunch of Voraatus' minions, who will slowly spawn in the room at the two sarcophagi situated against the northern and southern walls (#18).
If this all sounds like something you'd rather not deal with, then you can simply fight Voraatus. He's a big demon, he's tough, and a bunch of his minions will appear in the room to help him, but his battle is easier than the one with the Fiend of Ghazbu (#11), if you got to experience that. We'd recommend that you at least attempt to kill Voraatus, and if that doesn't work, then feel free to lose attribute points playing his game. Whichever way you end up dealing with Voraatus, you'll receive Vartugg's eye as your reward.
Note: You can also make your own deal with Voraatus. He'll let you pick one of your attributes, and then he'll add X to it (where X seems to be a number between 15 and 20). But then he'll remove X from your honor attribute, and he'll remove X more from your other four attributes combined. In other words, this doesn't seem to be a great deal, but if you have one attribute that is very high (and thus very expensive) it's possible the deal could save you some advancement points.
18 - Sarcophagi
The sarcophagi can't be opened.
19 - Watcher
After bargaining with (or killing) Voraatus (#17), this Watcher will offer you a reward, either the "demon's steel" or the "demon's spell." If you pick "steel," then the Watcher will open the treasure room to the right (east), and you'll find a darkwing helm, an angelic destroyer, and a blade of death, and you'll also be able to loot two chests. If you pick "spell," then the Watcher will open the treasure room to the left (west), and you'll find the nether spell "summon deathlord," plus be able to loot two chests. If you're playing version 1.4/1.5, then you'll also find the arcane spell "cataclysm" in the demon's spell room.
Note: As far as we can tell, this is the only place in the game where you can get the "cataclysm" spell. But if you sell the spell to a shopkeeper, then you'll be able to buy it back as many times as you'd like.