What (if any) mortal, non-devil creatures could a party encounter in the Nine Hells? Best I can discover in researching 5e material, the only creatures in the Nine Hells are:
anything tagged as a "devil," (or "demon" if they are part of invaders from Abyss),
a mortal's soul that arrived after death but those are just going to be worms or grubs in the Pit,
and other planar travelers like themselves (and surely there's not going to be many of those. Though the DMG does mention Dis as being a popular destination for travelers looking to make deals).
Is there anything else they could and would encounter? Any natural, mortal denizens of Nine Hells?
Night Hags also live in the Hells, so you could easily add covens of mortal witches to your campaign.
Realistically they can discover anything you might want--you are the DM, so the rules are yours to mess with. One example, you could build off of the Descent into Avernus plot, which, in part, focuses on a city dragged down into the Hells. Since this is evidently something that can happen, you can say this has happened before and either have a city that either was just dragged down, or had been dragged down generations before, resulting in an entire group of mortal descendants who are surviving in the only land they have ever known.
I think you first have to understand that the Hells aren't so much a natural ecosystem as a world more or less engineered by Infernal minds and wills (or a mind and will if you want to put Asmodeus on that pedestal). Avernus being the one exception as it was supposed to be a paradise but turned into battlefield waste land as it became one of the if not the major battlefield of 5e's interpretation to date of the Blood War (I'm not sure about other editions but 2e's Planescape treatment of the Blood War is much more complicated).
As part of your reading, in 5e I'd say Descent into Avernus should be looked at.
So the only thing "natural" to the Hells are the Devils (and they are constructed by intelligence and will of Asmodeus and other high ranking Devils rather than some "natural biological" reproduction.
The worm thing is Hades that's a different place. As far as I can remember mortal souls delivered to the Hells after death become Lemures, or maybe a higher level Devil (Tiamat's followers are sometimes rewarded with becoming an Abhishai for example). Descent into Avernus also introduces the concept of Soul Coins, where Mammon diverts some souls from Lemuredom and turns them into a form of currency in the Hells.
Everything beyond that was either allowed into Hell or found its way in. Abyssal Chickens are found and grown in at least Avernus, and that's a result of Abyssal incursions. Demons are found mostly on Avernus as that's where all Hell "holds the line", finding them at least in large numbers elsewhere in Hell would be unusual I imagine. Yuggoloths are used by mercenaries and have their own agenda during the Blood War, so they're found a lot on the battlefields of Avernus. So I think Avernus is the most "melting pot" of the Hells, simply because its a warzone and you have a lot of beings from all over the planes seeking out opportunities or otherwise having to enter the conflict space. The rest of the levels you could see other creatures of the Fiend category easily having been brought in for a specific plan and otherwise having made the plane part their home. Some layers makes a lot of sense to have fire giants around, others frost giants. Lots of uses for dragons in hell.
Keep in mind, Devils are literally bound to Hell, most if not all can't just leave. Consequently devils have uses for mortal and just non infernal agents to do work for them beyond the Hells and thus some of those agents likely report to hell as part of business. Devils could keep beings from other planes as bargaining chips in part of their dealing with mortals and others. Devils could keep pets and use some creatures as guardians for special locations where Devils are better used elsewhere or the Devil wants the rest of hell to be in the dark over what the non devil guardians are guarding (a difficult prospect).
Hell is a place bound by or assertive of rules. But as DM, you get to make those rules and populate Hell however you want. There's a lot of rich lore in the Hells in prior editions, though they may disagree with some elements of 5e. Yuggoloths for example have sort of been nerfed in their reasons for existence and their backstory flips the script of prior editions - in 2e Yuggoloths are said to be purer fiends than Demons or Devils (which come from mortal souls) and predate both Demons and Devils ... in 5e it's said the Yuggoloths were a creation of Asmodeus and a cabal of Hags over which they lost control.
I think you first have to understand that the Hells aren't so much a natural ecosystem as a world more or less engineered by Infernal minds and wills (or a mind and will if you want to put Asmodeus on that pedestal).
None of the outer planes are, though it's debatable whether the plane is shaped by the inhabitants or the inhabitants are shaped/made by the plane. The outer planes fundamentally have alignment, and alignment doesn't make sense in the absence of sapience.
I think you first have to understand that the Hells aren't so much a natural ecosystem as a world more or less engineered by Infernal minds and wills (or a mind and will if you want to put Asmodeus on that pedestal).
None of the outer planes are, though it's debatable whether the plane is shaped by the inhabitants or the inhabitants are shaped/made by the plane. The outer planes fundamentally have alignment, and alignment doesn't make sense in the absence of sapience.
It's a riddle unless you go full on Twin Peaks "We're all living inside a dream".
A throw away line I use a lot in my campaign when Far Realm types are encountered, "None of this is supposed to be here." So it's sort of the proverbial "things that should not be" saying, "No, you're the things that should not be."
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Jander Sunstar is the thinking person's Drizzt, fight me.
Though the DMG does mention Dis as being a popular destination for travelers looking to make deals).
this travelers in hell concept is in legacy 3e Manual of the Planes as well. I'd say in the big cities it wouldn't be unusual to see a traveler...or anyone else being tormented but not yet killed (i think of them as urchins...most other devils don't mess with someone being tormented as that could invoke the wrath of the tormentor as well...so they just let them wander about).
What (if any) mortal, non-devil creatures could a party encounter in the Nine Hells?
Best I can discover in researching 5e material, the only creatures in the Nine Hells are:
Is there anything else they could and would encounter? Any natural, mortal denizens of Nine Hells?
Natural? No, but Tiamat appears on the first layer.
According to I think the DMG, kobolds are mentioned as being found in Nine Hells. How the heck did 1/2CR kobolds find their way into the Nine Hells?
So I have to wonder, are there others and why and how?
Night Hags also live in the Hells, so you could easily add covens of mortal witches to your campaign.
Realistically they can discover anything you might want--you are the DM, so the rules are yours to mess with. One example, you could build off of the Descent into Avernus plot, which, in part, focuses on a city dragged down into the Hells. Since this is evidently something that can happen, you can say this has happened before and either have a city that either was just dragged down, or had been dragged down generations before, resulting in an entire group of mortal descendants who are surviving in the only land they have ever known.
I think you first have to understand that the Hells aren't so much a natural ecosystem as a world more or less engineered by Infernal minds and wills (or a mind and will if you want to put Asmodeus on that pedestal). Avernus being the one exception as it was supposed to be a paradise but turned into battlefield waste land as it became one of the if not the major battlefield of 5e's interpretation to date of the Blood War (I'm not sure about other editions but 2e's Planescape treatment of the Blood War is much more complicated).
As part of your reading, in 5e I'd say Descent into Avernus should be looked at.
So the only thing "natural" to the Hells are the Devils (and they are constructed by intelligence and will of Asmodeus and other high ranking Devils rather than some "natural biological" reproduction.
The worm thing is Hades that's a different place. As far as I can remember mortal souls delivered to the Hells after death become Lemures, or maybe a higher level Devil (Tiamat's followers are sometimes rewarded with becoming an Abhishai for example). Descent into Avernus also introduces the concept of Soul Coins, where Mammon diverts some souls from Lemuredom and turns them into a form of currency in the Hells.
Everything beyond that was either allowed into Hell or found its way in. Abyssal Chickens are found and grown in at least Avernus, and that's a result of Abyssal incursions. Demons are found mostly on Avernus as that's where all Hell "holds the line", finding them at least in large numbers elsewhere in Hell would be unusual I imagine. Yuggoloths are used by mercenaries and have their own agenda during the Blood War, so they're found a lot on the battlefields of Avernus. So I think Avernus is the most "melting pot" of the Hells, simply because its a warzone and you have a lot of beings from all over the planes seeking out opportunities or otherwise having to enter the conflict space. The rest of the levels you could see other creatures of the Fiend category easily having been brought in for a specific plan and otherwise having made the plane part their home. Some layers makes a lot of sense to have fire giants around, others frost giants. Lots of uses for dragons in hell.
Keep in mind, Devils are literally bound to Hell, most if not all can't just leave. Consequently devils have uses for mortal and just non infernal agents to do work for them beyond the Hells and thus some of those agents likely report to hell as part of business. Devils could keep beings from other planes as bargaining chips in part of their dealing with mortals and others. Devils could keep pets and use some creatures as guardians for special locations where Devils are better used elsewhere or the Devil wants the rest of hell to be in the dark over what the non devil guardians are guarding (a difficult prospect).
Hell is a place bound by or assertive of rules. But as DM, you get to make those rules and populate Hell however you want. There's a lot of rich lore in the Hells in prior editions, though they may disagree with some elements of 5e. Yuggoloths for example have sort of been nerfed in their reasons for existence and their backstory flips the script of prior editions - in 2e Yuggoloths are said to be purer fiends than Demons or Devils (which come from mortal souls) and predate both Demons and Devils ... in 5e it's said the Yuggoloths were a creation of Asmodeus and a cabal of Hags over which they lost control.
Jander Sunstar is the thinking person's Drizzt, fight me.
None of the outer planes are, though it's debatable whether the plane is shaped by the inhabitants or the inhabitants are shaped/made by the plane. The outer planes fundamentally have alignment, and alignment doesn't make sense in the absence of sapience.
It's a riddle unless you go full on Twin Peaks "We're all living inside a dream".
A throw away line I use a lot in my campaign when Far Realm types are encountered, "None of this is supposed to be here." So it's sort of the proverbial "things that should not be" saying, "No, you're the things that should not be."
Jander Sunstar is the thinking person's Drizzt, fight me.
this travelers in hell concept is in legacy 3e Manual of the Planes as well. I'd say in the big cities it wouldn't be unusual to see a traveler...or anyone else being tormented but not yet killed (i think of them as urchins...most other devils don't mess with someone being tormented as that could invoke the wrath of the tormentor as well...so they just let them wander about).
Guide to the Five Factions (PWYW)
Deck of Decks