The first premise being, that existence after death is not a stagnant and calcified experience: that, from moment to moment, events occur which provide opportunity for the creation of narratives and change.
And the second premise being that the motivations of demons and devils are not, in fact, known or understood, except by those who have had the opportunity to die and experience these beings as they function.
From this starting point, we can see the opportunity to create a campaign that begins with the death of the characters, moves through a set, planned adventure that is based around the culture of demons and devils, that in turn allows the characters to have ambitions of some kind that can be pursued. We do not need to be bogged down by limitations such as, the only reason for the existence of a devil is to torture dead beings. Nor that a demon's conversation is all pain pain pain all the time. We can surmise that these creatures, too, have aspirations, goals, ideals, dreams ... and in turn, let downs, disappointments, failures, despair and so on.
After all, Earth's theology was designed to scare the shit out of people to ensure obedience; it was not based on observation and experimentation.
What matters, however, is that we want to retain most of the scenes as proposed by Dante and other writers ~ hordes of the dead pushing down to the river, where they wait to be taken, across, the dead floating in pools of acid or fiery oil, the horrific lines of the dead being whipped down winding paths over precipices by laughing, gleeful devils ... all that has to stay because all that is compelling and distinctive of a world that is definitely not the prime material plane. All I am saying is that while being boiled alive, there should be some sequence of options available that makes it possible to be bumped out of the pit and into some other kind of frying pan, which in turn provides an opportunity to get into the line where the next punishment provides some opportunity for redemption ... of a kind. Let's say, an opportunity for moving up in the world.
Now, getting down to the structural framework of the underworld. Let me say up front ~ if you haven't taken the time to read Dante, this might be hard to follow. I'll try to give a brief synopsis of the bits that I need explained.
In the Canto III of the Inferno, Dante borrows heavily from the imagery of the Greek Hades. The river that Charon, a Greek figure, takes you the sinner across is the Acheron, again from the Greek. The dead wait on the shore, as they do in Hades. There's no Cerberus on the other side, or Greek monsters, but within the 1st Hell (canto IV), that Dante names Limbo, there are many intellectuals, mostly Greeks and Romans, who were good men who never wished malevolence on anyone, but because they could not know of God or believe in him, they're doomed to this plane. Mostly, they sit about and wait. For what is not made clear.
Taking up the problem I mentioned two posts ago, the "all myths are true" issue as Tim calls it, part of the practical solution is to recognize that we as a people tend to call the same thing by more than one name. I think it is reasonable, then, to establish that Limbo and Hades are fundamentally the same place. Since we have no alignment straightjacket to adhere to, we don't need an arbitrary plane to exist between Pandemonium and Gladsheim. We have a geographical placement for Limbo in Hell, and therefore Hades is folded into Hell as well. Hades is simply the Greek name for the planes that they knew about.
We could argue that "Hell" was an extension of Hades that was conjured into existence by the gained beliefs of millions of Christians as their numbers swelled in the 4th century. I have, in the past, argued that "belief" fuels the powers of Gods; perhaps that, in turn, fuels the creation of physical spaces. As the Gods grow more powerful, they naturally start creating homes: and as Christianity expanded to be much bigger than the comparatively small number of Greek believers in their gods, the former plane became crowded and needful of renovation. Hell could have begun as an add-on of a level or two, then the expansion of eight new levels to comprehend the sudden complexity of what places needed to be made for which dead. Just a thought.
As an aside, I had considered making a map to add to this post ~ but I must confess, I haven't the energy or the wherewithal, at least not now. Perhaps another day, when I will add it to the wiki. Right now I just want this straightened out in my mind so I can get back to describing demons and devils.
Gehenna, then, is the Jewish destination of the wicked ~ and apart from many arguments about Gehenna referring to real places attached to the Holy Land, the part that matters to me is the rabbinical tradition that those condemned to Gehenna are made to spend no more time there than the period of one year (the Jews clearly being more merciful to bad people than the Christians). This, to me, sounds more like Purgatory ~ and that works for me, because the original map of the outer planes never did include a Purgatory in their conjurings.
Unfortunately, Gehenna cannot be reconciled with Dante's Purgatory as easily ~ and I like the Purgatorio. When Dante arrives in Purgatory, he and Virgil find themselves on a beach next to a big ocean, where occasionally boats come along and drop off the dead. These dead step off the boats singing about the escape from Egypt-land, so that at least gives us a strong relationship to Jewish mythology. We can, from there, relieve Gehenna of its flames and equate it to this initial part of Purgatory.
Dante calls it ante-Purgatory, the three levels before Purgatory, where the excommunicated who waited until the very last moment of their lives to repent are waiting on the arrivals beach. We can always argue that the sun is hot, sort of like burning, but that's not confirmed by Dante. Oh well. It gives us a place for Gehenna, and we can always say that Dante was misinformed. The main thing is that the repentant bad people have a zone between Hell/Hades and Purgatory where they can go and spend a year (or longer), giving us a bit more structure.
The Abyss is easier. It, too, is a Jewish legend, perceived as a part of Gehenna, beneath the ocean and sealed. It is the seat of the evil spirits, that being demons. It is equated with Sheol, also a very deep pit, repeatedly mentioned in the Bible and likely derived from an Assyro-Babylonian word, "Shu'alu", where in that culture is the place where the dead are cited or bidden. We can assume, then, that there are a lot of different names for this same place. We can imagine that the ocean that this Abyss is under is the same that the beach in ante-Purgatory/Gehenna looks out over, that the boats cross.
Perhaps souls that do not properly get off the beach feel compelled to dive into the sea and thus disappear forever, into the Abyss. Perhaps the arrival at Purgatory isn't a guarantee of eventual redemption. The boundary might be fuzzier than we imagine, and that actions that are taken after death have influence on where we go, as well.
Now, Tartarus is also an abyss. It is where the monsters go, where the bad gods go (the Titans and such), it is the primordial place where the Greek mythology of the present imprisoned the mythological ideas of the past. Tartarus is, therefore, old, much older than any of the other places in the underworld ~ old enough for Cthulhu, perhaps, and gods older than Kronos and Uranos before him.
Geographically, Tartarus is viewed as "below Hades" ... which makes a connection between it and the rest of our model, as Hades is the top of Hell. I'll go one further and argue that Tartarus is the Abyss ~ one and the same, just as the Abyss is Sheol and Shu'alu. This gives us a passage from Hades/Hell into Tartarus/the Abyss and a passage from Gehenna to same. This might be a passage out or a passage in.
We might imagine our dead party, arriving at the Acheron, told to find their way into Hades, to avoid the deeper Hell, find the terrifying passage into the Abyss, avoid the much older passages into Tartarus under the Abyss, in order to find their way out of the Abyss and onto the beach of Gehenna, where my some means the might climb up the mountain of Purgatory and, perhaps, find a way into life? Perhaps there is a passage from the top of Purgatory across Eden into the Prime Material, before moving onto Paradise. Only the DM would know.
This, then, is enough for now. I'll just add that I feel that demons and devils are, in fact, divine beings. That some may be twisted, as Ozymandias suggests ~ but then again, some may not. Some might be disposed to help a party trying to find the passageway out of Hades or out of the Abyss. Though telling them apart from the "helpful" demons who are directing the party down the wrong passageway ... that might be difficult.
Oops, forgot Pandemonium. Well, another time.