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1 - GATEHOUSE
Before you enter the Hell plane, you make a short trip to the Gatehouse, where the Reaper fills you in on what's going on. He tells you that if you can find out what his True Name is, you can go back to Toril and give Mephistopheles a spanking. He will also bring back any of the henchman available in the Underdark if you want.
2 - PLANAR GATE
City of Lost Souls
You get back to the Realm of the Reaper through this gate now, instead of your relic, which Mephistopheles kept. Equip yourself with element resistant items, because it's so cold on this world that you'll intermittently take damage.
3 - ICE QUARRY
This quarry provides the means of punishment for the souls trapped here, and provides ice for Hell's capital city. If you want the demon to sell you items, you'll have to pull an imp out of one of the grinders (after a dexterity check) then repair it (an intelligence check). Alternatively, you can solve the problem by sending the demon through the grinder, for five points toward evil. The imp will tell you that the two people most likely to know the Reaper's True Name are Mephistopheles himself and the Sleeping Man (the oldest resident of this Hell Plane). If you come back to the quarry wearing the Sleeping Man's Ring (before you enter the Astral Door outside the temple), you can open an Astral chest for some goodies, including "The Iron Skeleton", a good suit of armor.
4 - BAR BRAWL
A guy named Ardin Swift is hanging out at this bar. He says he will give you answers for money, but he really just takes it. Kill him to get his trumpet, which you'll need to wake up the Sleeping Man. Alternatively, you can play a game with him to get the trumpet without violence. He'll pick a card from an ordinary deck and give you five yes or no questions to figure out which one it is. The tricky part is that his answers are often lies and you won't know whether they are or not if your wisdom score isn't high enough. Of course, with a high enough wisdom score, you can interpret the Sleeping Man's dream and open up the door to the room with his ring without waking him, anyway. If you are able to fight people who conceal themselves fairly well, killing Swift is much less time consuming than playing his little game.Pickpockets can avoid it as well, if they're skilled enough.
Rizolvir's ghost is also hanging out in the bar. He has a smithy in the back and will offer you the same sevices he did back in Lith My'athar. If you come back to his area wearing the Sleeping Man's Ring, you can get the Black Bow of Baator from an Astral Keg.
5 - SLEEPING MAN TEMPLE
The Sleeping Man resides at this temple. In order to get access to him, you'll have to answer five questions about him (or be skilled enough to unlock the door, or evil enough to want to kill the sensei). You can't do this by trail and error, though, so you'll have to search around for the answers. The caretaker will tell you the answer to the first question, and you can read her book after she leaves for the answer to the fourth one. The sullen pilgrim outside knows the third mystery, so you really only have to hunt for two. Once you've found all five, come back and recite the five mysteries for the sensei. She'll give you an amulet that you can use to see the Sleeping Man's dreams if your wisdom is high enough. Wake him with the Pandemonium Trumpet (or cut him if you don't mind moving toward evil) and he will open a door to a vault, where you can get a ring that will lead you to the Keeper of Places. You're supposed to ask her where to find the Keeper of Names, who will know what the Reaper's True Name is. When you get the pieces of the ring (from the three pedestals in the vault), "use" one of the ring pieces, then put them together in this order: "H","F,"L". Put the ring on in the vault room to find some hidden chests, one of which is only accessible once you've found the demon grappling hand later on (it contains a Cloak of Kings, which gives you -4 to Charisma, but +2 to all your other stats and makes you immune to cold). Wear the ring in the room with the Sleeping Man to find an altar that has a very useful helmet and some other trinkets.
6 - CAVE
This cave is magically sealed to you at the moment, but if you help the Scrivener sitting outside it, the seal will be lifted. The Scrivener needs to write the name of the person in the cave onto a wall in the town, but you'll have to lead him past several landmarks to help him find it. He'll tell you which pillars you need to take him to through word pictures. The first one is of a tree and a man holding a male child, so the answer is "Treason". To spare you the rest of these puzzles, which do get worse, lead him to "Treason", then "Dimension", and finally "Infinity". Once he's written the name on the wall, you can get into the cave. You can also find out the answer to the second Sleeping Man question from the Scrivener.
7 - FORTRESS OF SOLITUDE
Inside the cave you'll find none other than Lady Aribeth herself. Unfortunatley, she's encased in ice, so you'll have to find a way to light a fir that's warm enough to thaw her out. You need to go around picking velox berries until you have three of them. Use them to make a fire and she'll thaw out. She will attack you at first, but once she's beaten she'll talk to you, most notably telling you the answer to the fifth Sleeping Man mystery. If you can convince her to ask Tyr for forgiveness, she'll come along as a henchman.
8 - ICE CRYSTAL
If you look in this ice crystal, you'll see what Mephistopheles is up to. Remember that army of drow and duergar and such that you killed to get to the Valsharess? Well, they're reanimated now and working for Mephistopheles.