What's Hell like?
First, I would suggest that you consider where the PCs begin their sojourn in Hell. It really depends on which layer and, in some cases, which areas of which layer.
Some people have offered some suggestions for each layer, but I'll expand on them.
Avernus the First: Who rules depends on which source you use. If you're using official WotC stuff, the ruler is the former Pit Fiend Bel. If you're going with older TSR material (1ed only), the ruler is Tiamat the Queen of Dragons. Green Ronin Publishing has a devl name Baal ruling. In the case of Bel and Baal, Avernus is essentially a huge military encampment. I would imagine infernal checkpoints, but I would add additional elements. For example, perhaps there are tests to ensure the nature of those passing through. Some may be simple
true seeing type tests, while others may involve behavior. In any case, there are going to be a lot of patrols and the likelihood of interlopers either being conscripted into military service (and likely tortured or brainwashed) or having to make huge bribes to those in charge. If the ruler is Tiamat, then there are going to be multiple fiefs spread across the layer, many ruled by deposed Dukes of Hell and Greater Devils (Cornugons and Pit Fiends most likely). This could require real thinking on your part to come up with the different kinds of politics involved with these vying factions as all hate the current noble hierarchy in which they've been deposed. Also, in this version you're more likely to run into more Dragons. In any version,
fireballs, rivers of blood, and maggot holes are common. Dominant Devils in most cases are Abishai, Lemures, and Barbazu.
Dis the Second: This is the home of the largest city in Hell. Bear in mind that reaching here requires that those without clearance from this layer's ruler, The Arch-Duke Dispater, must enter by going through Tiamat's Layer... That could be an adventure in and of itself. At any rate, most sources agree that Dispater rules here and that he's extremely paranoid and rarely leaves his Iron Tower, a huge iron structure visible from anywhere in the layer. Furthermore, since Dis is the city in which mortals are "welcome," it's also the place where deals are most likely to be made. Therefore, it makes sense that Erinyes, among Hell's most prolific tempters, are the dominant Devils. This is probably the safest place in Hell physically despite the burning iron paths and dagger-like, but the most dangerous spiritually because it's all about deals and flawed/corrupt ethics. Everything's an illusion and a lie here. In many ways, Dis is probably the most fun place to start a campaign since PCs will always be second-guessing themselves. Furthermore, Dispater himself is very much aware of everything that takes place in this layer, but he's not the type to lay waste to mortals, but toy with them. Even mid-level PCs could interact with him. I strongly recommend that you check out 2ed's
Fires of Dis for more info on Dis.
Minauros the Third: This is a stinking, sinking bog of a layer. It's personified by the greed that weighs down the soul. Many are lost in the bog, desperately trying to escape with their ill-gotten gains. The layer is ruled by Mammon, the Count of Minauros. This is essentially a place where a lot of action can happen if PCs find themselves unwelcome since the Devils here make sport of hunting down those lost in the layer. Additionally, if the PCs have lucrative items, Mammon or one of his Dukes may well decide to tax them.
Phlegethos the Fourth: This is the layer of diabolical fire. It is the layer of pain and suffering. This is where things essentially go down hill... assuming they could get worse. This layer is ruled by Fierana and her father, Belial. Now, bear in mind that this is the current set-up for WotC. Older TSR has just Belial ruling, as does Green Ronin Publishing. This is where Devils undergo torment and purification and where Pit Fiends are said to be formed. I also suggest that this is where new souls arrived to Hell form from the the Pit of Flames near the center of the Layer.
All manner of torture and torment are practiced here, particularly those that scar the body. Neither Fierana nor Belial are likely to use mortals for anything other than sexual puppets, burning their husks in the process. Imagine the Elemental Plane of Fire and make it worse, and you have Phelegthos to a "t." Also remember that the head of security for the first four layers, the Pit Fiend Gazra, commands a few companies of Hamatula Devils. Essentially, PCs aren't likely to go unnoticed here regardless of their power level unless they take supreme precautions.
Stygia the Fifth: This is sort of like a warped Elemental Plane of Water... and all of it is composed by the River Styx, the touch of which causes most entities, including gods, to lose their memories. Chunks of ice float all about. Who rules this layer depends on which source you go with. In the case of old TSR, it was the Great Beast Geryon. According to both 2ed and 3ed, Geryon was cast out of the rulership of this layer and replaced by a long and still imprisoned Arch-Devil named Levistus. Levistus is still trapped in center of a huge iceberg for a crime he committed against Asmodeus. Green Ronin Publishing uses Leviathan, who's the largest sea monster ever known. No one's really bothered to describe this layer much over the years. If you want more information on my take, feel free to ask.
Malbolge the Sixth: This is another one where rulership changes from source to source. In old TSR, the layer was ruled by Moloch, a vassal of the next layer's ruler, Baalzebul. Since 2ed edition WotC, the layer's been ruled by The Hag Countess, an ascended Night Hag and former lover of Moloch who played a role in his being cast out by Asmodeus. Green Ronin and your's truly use Lilith as the ruler of layer. Traditionally, the layer is a perpetual avalanche of rocks and storms. There are various fiefs spread across the layer and there are said to be places where even Pit Fiends fear to tread. Again, there's very little "official" info on this layer.
Maladomini the Seventh: In all cases, this layer is ruled by Baalzebul (or any number of other, similar spellings). It's his background that's disputed. Current WotC and Green Ronin state that he's a Fallen Celestial of some sort. Old TSR treated him like another Devil. In anycase, this layer is a place of intrigue and subterfuge, where nothing escapes the multifaceted eyes of Baalzebul, the Lord of the Flies. Hundreds of huge, dilapidate, ancient cities dot the layer, most of them left unfinished at different stages of construction. Baalzebul would be aware of PCs presence and would likely have them interrogated shortly upon their arrival as he suspects all he doesn't invite as spies from Mephistopheles or Asmodeus.
Cania the Eighth: Mephistopheles rules this layer. Between him and Baalzebul, most of the intrigue that infests Hell comes from them. In the past and in Green Ronin, Mephisto is a dark, cold lord. In 3ed WotC, he's the lord of contradications, living as a burning furnace within a layer of ice. Cania is essentially a diabolical version of the Paraelemental Plane of Ice. There are many different beings buried under the ice of Cania, including Celestials and nameless creature. Mephisto is the Devil closest to Asmodeus in power, but has historically struggled with his various vassals just to maintain Cania. There is but one way to Nessus and it's through Cania's Pit (don't remember the precise name). Guarded at all times by 9,999 Gelguons (or some similarly outragous number), none may enter it without Asmodeus' express approval.
Nessus the Ninth: The heart of LE, this layer is essentially not well known to most beings, mortal or not. The layer is dominated by Pit Fiends and the largest structure in the Outer Planes, the fortress Malsheem, is where the Dark Eight meet under Asmdoeus. What Asmodeus is depends on the source. In TSR, he was the most powerful Arch-Devil who, like the other Arch-Devils, was a lesser god. In 2ed, Asmodeus was a fallen Greater god whose true form, that of a tremendous serpent, is trapped at the bottom of a huge chasm. The Asmodeus seen by most is just an avatar. 3ed has taken something of a middle ground on both ideas, providing "traditional, non-divine" stats for Asmodeus while suggesting that he may well be something more than a "regular" Devil. Green Ronin explicitly states that he's a fallen god of at least Greater status.
Hope this list helps. Our site, Dicefreaks, has information on the Hells and its denizens. Feel free to check it out.
www.dicefreaks.com.