https://forgottenrealms.fandom.com/wiki/Abyss
Opening Thoughts:
The Abyss has been around pretty much as long as D&D has been a thing. Drawing from... well, I'm actually not sure what it precisely draws upon; I always pictured it as the more pulpy weird fantasy "generic hellish netherplanes" plane, in contrast to the more Dante's Inferno-inspired faux-Christian Hell that is the Nine Planes. Regardless of where it came from, it serves as arguably the most iconic of the fiendish "lower planes" in D&D's cosmology.
The Abyss:
In the book, the Abyss is not given a separate chapter, but is part of the Elemental Chaos. The precisesentence is that the Abyss represents entropy and annihilation, and consequently the demons that inhabit the Abyss are extensions of this cosmological niche. In other words? Demons in the World Axis are elementals of corruption and destruction; manifestations of the darkest concepts that exist to go forth and tear down everything else, until all has been swallowed in the Abyss and ultimately obliterated. They hunger to slay the gods themselves, believing that doing so is the key to the annihilation of all things, but such a feat is currently beyond them; hence they focus their depredations on the mortal worlds, hoping to weaken the gods by ruining their most prized creation.
The origins of the Abyss are an in-universe theory. Some say it was created, either by the gods or something "even older", as a kind of cosmic vacuum - that it exists to consume the excess elemental material endlessly spewed forth by the Chaos. Another theory is that it sprang into its existence simultaneously, a natural countermeasure to creation. Others believe that the Abyss is a sentient force, with demons being merely extensions of its unfathomable caprice for life and the existence of anything aside from itself.
And then there's the tale told in the subchapter below...
Birth of the Abyss:
This myth describes a god whose name has been forgotten; a figure who lusted for power absolute before the Dawn War had even erupted. Seeking a way to gain absolute control over the nascent universe, he sought a weapon that would give him the requisite strength, and found a tiny shard of utter evil.
When he touched it, he was consumed; this god is no longer recognizable as the being he once was. Driven by alien impulses, he fled into the depths of the universe, the lowest reaches of the Elemental Chaos, and planted the seed there. Its dark influence blighted the very nature of the plane around it; an enormous necrotic wound sprang forth, one that I believe a later source will describe as "big enough to swallow galaxies".
Thus was born the Abyss.
Primordials in its area of influence were transformed into the first Demon Lords, creating the most elder and fearsome of those creatures - Demogorgon, Baphomet, and Orcus are all cited. The mad god hoped to use these beings as weapons in a war of universal conquest, but they would not heed him. So he abandoned them and marshalled other elemental forces instead.
In the end, this mad god was overcome, and chained forever in a secret place. But whilst the so-called Elder Elemental Eye is bound forever, the Abyss remains; it continues to fester, spawning demons and hosting demon lords, which consume each other in endless bloodlust even as they dream of reaching forth and tearing down everything else. The goddess Lolth hides here too, corrupted and perhaps driven mad by the same power that shattered the Chained God and made the first demon lords.
And somewhere far beneath all imagining, the crystalline Heart of the Abyss still beats its unceasing cadence of evil.
Demons:
This subtopic starts talking about what defines demons in the World Axis. It acknowledges that their characterization as primordial, destructive creatures isn't new, but notes that the emphasis on this element is new. It describes demons as beings fundamentally born to annihilate; they literally can't rightly comprehend the desire to create. This fuels their enmity for the gods, who in a sense exist to create, and likewise drives them in their attacks on the mortal realm - it's easier to reach. That's not to say that mortals can't strike bargains with demons, but such pacts depend on the ability of mortals to offer alternative targets for the demonic appetite for destruction.
The topic then takes a meta twist, explaining that demons are visually characterized by their more monstrous, malformed forms compared to the comparatively smooth and human-ish frames of devils - "humanlike" features are relatively few and abstract, compared to how devils would look more like funny-colored humans with added bits. They will also be mechanically defined by a focus on offense over defense.
It then switches back to lore, explaining that whilst the demon lords are powerful, they are still "only demons". The reading I take from this is that as powerful as demon lords are, they still will typically pale in power compared to a primordial or a god. Their main advantage is numbers; one demon lord may not be quite a match for one god on its own, but there are many, many more demon lords that there are gods. This numerical advantage compensates for the natural anarchy that plagues demonkind and prevents them from readily working together, and is called out as why the gods prefer to simply contain the Abyssal threat rather than try and expunge it - in fact, some believe that an open war against the Abyss would be futile, as it would simply spawn new demon lords as fast as old ones are killed.
Demon Lords:
This subtopic is an adress to reading DMs that 4th edition respects the demon lords and their heritage as the most iconic of D&D's "boss monsters", and seeks to maintain and promote them in that field, but still also simplify them to use in play, compared to 3rd edition and their incredibly long laundry lists of spells and special abilities.
Demons Reconcepted:
Demons and devils have been a part of D&D since the beginning, and as long as they've been around, people have been asking one simple question:
Why do we have them both?
The sad truth is that demons and devils are one of the oldest and most iconic victims of the alignment-based bogus parallelism that the Great Wheel wrestled with. Many casual fans could not really differentiate the two to any major degree. Although 3e attempted to better differentiate the two of them with its Fiendish Codex duology, the 4e designers decided that now would be a golden opportunity to work on finally making a major differentiation between the two.
This is the list of traits that Scramjet came up with to fully define who demons are a monster in the World Axis:
- merciless, savage, hateful destroyers
- varied but universally monstrous in appearance
- fearless
- eager to destroy the creations of the gods
- difficult to enslave
- disorganized
- self-destructive
- prone to gathering in hordes
- not empire builders
- not interested in negotiation
- not interested in personal power, wealth, or prestige
- more inclined to use melee attacks than ranged attacks
- more dangerous when injured
From this, demons and devils moved in different directions both in lore and in mechanics. Devils became fallen angels cursed for betraying their divine master under the rulership of Asmodeus and his archdevils, with Baator now being a twisted dominion within the Astral Sea. And demons, as discussed before, became elementals of corruption and ruin.
The topic concludes by noting that an angle that 4e wanted to go with for demons is that they become increasingly vicious and frenzied as they take damage. They were also, at the time of writing, exploring a "variable threshold" mechanic, which would allow demons to gain resistance to damage of a chosen type a limited number of times per day - ideally, this would make them more tactical to play against and also reinforce their mercurial motif.
Behind the Scenes: Who Switched Sides:
This sidebar ties into the aforementioned subtopics, and basically repeats what I just said: demons and devils overlapped too much in the Great Wheel, so the Scramjet team wanted to change that. In particular, they call out the amount of "bogus parallelism" - many fiends in the Great Wheel are literally filling the same role, but for the different teams, such as pit fiend and balor or succubus and erinyes. The rules to distinguish them were often superficial or arbitrary, and even worse they were ocasionally inconsistent - for example, some demons such as dretches and mariliths are vulnerable to cold iron weapons, whilst other demons aren't.
As a result of creating a clearer line of distinction based on physical characteristics, roles and behaviors, it became apparent to the team that some monsters deserved to switch factions. The most iconic example (in fact, I can't name any others!) is the succubus, who went from a demon to a devil because her modus operandi of seduction and temptation meshes better with the World Axis depiction of devils, and her visual style of "humanoid with monstrous traits" likewise matches this new cosmology's diabolic motif.
It concludes by reassuring readers that they're not just arbitrarily changing demons and devils around; their first act is instead to try and find ways to make a given demon or devil feel better suited to its faction and less of a "here to be a counterpart to some fiend on the other team". Cases like the succubus are simply exceptional.
Yugoloths Reconcepted:
Our final subtopic for this subchapter concerns the yugoloths; the little-remembered Neutral Evil fiendish race that Planescape tried so hard to sell us all on. With the alignment grid dropped and the Blood War downplayed as a major cosmological concept, yugoloths as they were no longer fitted into the World Axis.
Instead, they have been changed to a strange, relatively new strain of demons - one characterized by their surprisingly nuanced goals and their greater willingness to cooperate both with each other and with the mortal races. They have an unusual mercenary streak, and are naturally more disciplined and coordinated, which mechanically manifests as a preference for the Soldier monster role over the more demon-typical Brute. In short; they're the most devil-like of the demons.
In its final paragraph, this decision wasn't as arbitrary as you might think. Yugoloth names have always had a more "demonic" feel to them, and when they were introduced, it was in 1978 as allies of the demon-worshipping drow. So changing them into demons is not so much a drastic new step as taking them back to their origins as the "daemon" subtype of demon.
I do think that dropping the -loth suffix in favor of the -demon suffix was a mistake, though, because names like mezzodemon and nycademon just feel more awkward to me. I don't know why.
Closing Thoughts:
To be perfectly honest; I've never really liked the Lower Planes of D&D all that much. Baator, the Abyss, and the rest of the Great Wheel's netherrealms just never really drew me in, and that's in no small part due to how bland I found both demons and devils to be, mainly because they tended to just blur together in my perspective.
My preferred netherrealm is, ironically, a 3rd party depiction; the Infernum setting by Mongoose Publishing, which I feel can easily slot into the World Axis and replace both fiends altogether.
Still... I definitely feel the World Axis is a marked step up from what came before, and I actually like the backstory that the Abyss has as a literal festering wound in reality itself. It's much easier for me to get invested and find interesting ideas to do in the World Axis take than in the Great Wheel, even if I do think that the Wheel did come up with a lot of theoretically interesting layers that could have used a 4e spruce-up.