Underdark

Not a bad place to live, if you consider the scenery.

The Underdark is a Dungeons & Dragons term for the sprawling cavern-networks under its settings. The concept was already roleplayed in 1970s-era networked-computer games like Advent and Zork. Gygax in the Drow series brought it to Greyhawk, though not giving these realms their catchy name. That would happen with Douglas Niles' 1986 Dungeoneers Survival Guide - actually, with two names, alongside Deepearth. The Forgotten Realms went with the latter.

Niles was already aware of Hollow World theories, such as Mystara would later moot: the Underdark is "just" caves, rather than being lit by some sort of internal sun. Of course, calling the Underdark a "cave system" is like calling the God-Emperor of Mankind a "powerful psychic": technically true, but doing an injustice to the scale of the thing. The Underdark stretches for absolute bloody miles in all directions, reaching down to the heart of the planet and sprawling under continents and seabeds, allowing a cunning, powerful and determined adventurer to get to any continent on the planet if they want. It's big enough that it's an entire ecosystem unto itself, with rivers, lakes, abysses, hills, and all of the other geological features we know and love from the surface world, all with a stone roof overhead.

Notable Denizens

The Underdark is full of life, and pretty much all of it is hostile. All kinds of gribblies wander about here, from dark-preferring denizens of the world above to aberrations boiling out of the uttermost depths. These range from mindless monsters like the Cave Fisher and the Carrion Crawler to sapient races.

Playable

• Drow
• Duergar
• Svirfneblin
• Derro
• Kuo-toa
• Troglodyte
• Rockseer Elf
• Urdunnir Dwarf
• Orc
• Goblin
• Orog
• Norker
• Chitine
• Munavri
• Xvart
• Baphitaur
• Vril
• Quaggoth
• Skulk
• Diopsid
• Sapromneme
• Zygodact
• Gruwaar
• Gloaming
• Slyth
• Jermlaine
• Dark One
• Deep Imaskari
• Lerara
• Underfolk

Monstrous

• Cave Fisher
• Carrion Crawler
• Illithids
• Slime
• Choker
• Aboleth
• Beholder
• Myconid
• Ghoul
• Fomorian
• Incunabulum
• Girtablilu
• Bainligor
• Grue
• Cloaker
• Shadow_Dragon
• Purple/Deep Dragon
• Adamantine Dragon
• Skum
• Draegloth
• Chuul
• Stone Giant

Splats

Naturally, being such a prominent feature of the D&D core identity, the Underdark has had a number of splatbooks dedicated to it. Due to its strong association with the Forgotten Realms, most of these have been released under the Realms' banner.

Advanced Dungeons & Dragons saw two such books; Drizzt do'Urden's Guide to the Underdark, which explores the geography and races of the Underdark as a whole, and Drow of the Underdark, which explores the drow society of the Forgottent Realms.

3rd Edition also saw two such books. Weirdly, whilst Underdark was essentially a 3e update of/analogue to Drizzt's Guide, the splat titled Drow of the Underdark was an extended look at drow culture as it applied to the drow of the generic setting of 3e (aka, Greyhawk with the serial numbers half-filed off).

4th edition released only a single Underdark splatbook; titled, simply, Underdark (yes, these naming conventions can get fucking confusing), it was an extensive look at the geography, history and races of the Underdark of the Nentir Vale setting - that is, 4e's generic setting, even including stats for Torog for epic-level spelunkers who want to claim a god's head but think the Demonweb is for amateurs. Ironically, playable versions of common Underdark races - namely goblins, kobolds and svirfneblin - wouldn't appear until the later Dungeon Survival Handbook.

The Darklands of Pathfinder are detailed in Into the Darklands (back when it was a 3rd Party 3E setting) to acompany the articles in the Second Darkness Adventure Path. This was later expanded in Darklands Revisted and Heroes of the Darklands.

Faerun's Underdark

There's some surprisingly nice places down there.

As mentioned, the original "Underdark" underlaid Greyhawk and Niles' version didn't belong to any trademarked setting - even as late as the first half of the H series. When Forgotten Realms took off as TSR's baby, they strongarmed Niles to migrate H2's "Deepearth" to Faerun. It would become iconic to the setting, in no small part because of the Drizzt novels, and indeed it's sometimes considered more of a Forgotten Realms region than a D&D region as a whole - the 3e "Underdark" sourcebook was even released under the FR banner.

When adventures in the Underdark first debuted, a special proviso was laid on most of the items found therein: most magic items found there became normal or disintegrated about a month after leaving the Underdark, which was some of the most insulting cheese that players had ever eaten. This was later justified as only applying to drow items, as they used a special metal that took to enchantment more readily, but which disintegrated upon exposure to sunlight. Still, the idea infuriated players to the point that this idea was ultimately dropped.

In the Realms, whilst most surface-dwellers only think of the Underdark as a single uniform entity (i.e. a massive cave system where nasty monsters come from), it can actually be divided into three general depth zones based on the conditions and inhabitants:

• The Upperdark or Upper Underdark is the region within approximately three miles of the surface. It's got most of the creature comforts of the surface (as far as the availability of food and shelter go), just a little dark and scary in places. Most traffic with the surface happens here, so most of the creatures living here are, well, nobody's really friendly down here, but at least not immediately hostile. Dwarf kingdoms dig down into this layer to expand; goblins and orcs set up camps and settlements here; they may also be brought here by factions that enslave them.
• The Middledark or Middle Underdark occupies the earth between three and ten miles of the surface. It's too far down to get regular circulation of air and water from the surface, so the ecosystem is poorer for would-be adventurers; what oases and natural resources do exist are all claimed and guarded. The Drow rule here, and the largest Duergar and Mind Flayer settlements can be found here as well.
• The Lowerdark or Lower Underdark is everything below ten or so miles down. This far deep, the Underdark is not nearly as interconnected, so it is much more fragmented into regions that can only be reached from the Middledark, teleportation, or other roundabout means. Most people from the surface have no interest in coming down this deep, and most creatures that live down here have no interest in going to the surface, which is good, because things get really weird this far down. Aboleths and Beholders are the dominant creatures, and the unwary adventurer may find herself falling through a portal to another plane, especially the Plane of Shadow. These portals become more frequent as you go down, leading some to suggest that the Lowerdark has no bottom, and simply becomes the Plane of Shadow. This probably isn't true, but nobody's ever mapped the Underdark to the bottom, so we can't say for sure. On the plus side, there are the Desmodus, a.k.a. bat-people (named for the Desmodus genus of bats, more commonly called "vampire bats"), who are actually nice, if isolationist.

There's also another place even further down called the "Utter Dark" which even the Drow dare not to tread. No light mundane nor magical has ever reached this place. This isn't your average everyday darkness. This is... advanced darkness. If you manage to last 30 seconds down there without being slain by the madness of complete darkness, then you get perfect darkvision for a year. Good luck clawing through three levels of perfect hell first to reach it though.

Nentir Vale's Underdark

4e will never live down some of its naming conventions.

When Dungeons & Dragons 4th Edition rolled around, of course they were going to include a version of the Underdark. The basic concept remains untouched - a big cave system full of gribblies - but there were some tweaks.

For starters, 4e decided to ask themselves just why the Underdark exists in the first place. Their answer? During the Dawn War, a god named Torog went down there and picked a fight with a Primordial named Gargash, who was experimenting with the concept of "imprisonment". After a brutal fight, in which Torog was hideously maimed, the god triumphed - only to find himself cursed. First, in that his wounds would never heal, and secondly, in that he could never leave. Torog literally carved the Underdark out of the bowels of the earth, forcing some level of divine stability upon a region that had once been like the Elemental Chaos in miniature, rending what is now called the King's Path in his search for freedom. And then, when he found the surface, he found that the further he got from the depths, the weaker he became; he has no choice but to remain in the deep in order to preserve his godhood.

The Underdark of the Nentir Vale is almost like a demiplane (or, rather, transitive plane) in its own right; its tunnels stretch through the ground of not just the Prime Material, but also the Feywild and the Shadowfell as well. In the deep lie places of barely-checked elemental energies, which can open to the Elemental Chaos. In the very deepest places in the Underdark, the substance of reality frays and degrades, chafed by constant scraping against jagged nothingness. And in those places where the world breaks down, the Far Realm breaks through.

Needless to say, if you have the guts and the strength, you can get almost anywhere here.

The Shallows

This region lies the closest to the surface, as you can tell from the name. As a general rule of thumb, if it's within 2 miles of the surface, it's considered part of the Shallows. The major sapient races found in this region of the Underdark are Duergar, Dwarves, Goblins, Humans, and Troglodytes, with Bullywugs and Quaggoths forming small but notable enclaves as well.

The Deeps

This covers every other part of the Underdark as lies beneath the World, without straying into the various other planes. This is the Underdark that everybody thinks of when the term is used, and thus it has the truly iconic populations; Aboleths, Beholders, Drow, Grimlocks, Kuo-toas, and Mind Flayers. Of course, there are plenty of other races down here; this is where the Duergar prefer to maintain their fortress-cities (their mines and slave camps are concentrated in the Shallows), then there are dragons - mostly Adamantines and Purples - and aberrations like the swordwings and balhannoths, Umber Hulks, Purple Worms and Hook Horrors.

The Feydark

The Underdark of the Feywild is rich in primal energies, making it full of life compared to the Underdark proper. Food and water abounds here - but so do vicious predators. Whilst teeming with all manner of beasts, vermin, plants and even fey, the major powers are the Cyclopses, Fomorians, Gnomes, Myconids and Spriggans.

The Shadowdark

The Underdark is nasty. The Shadowfell, as the land of the dead, is also nasty. So, when you combine the two, nothing good happens. This is one of the most lethal places in the multiverse that isn't the Abyss or the Far Realm.

It also has one of the most infamous names in all of the Nentir Vale, a source of so much mockery that the 4e Underdark sourcebook opens its dedicated chapter by listing alternative names, such as The Black, the Deep Chill, the World Tomb and the Soul Cold.

Although not exactly brimming with life, there are creatures and even civilizations hidden down here in the cold, dark depths. The Undead are the most frequent and powerful, of course, but there's also the Dark Ones and the Incunabula. Even the Shadar-Kai have been known to enter the place on exploratory missions (or raids, but they're practically the same thing), though they usually don't seek to establish a long-term presence here.

Other Settings

• In Mystara, you have the Hollow World, which is a pulp styled "land that time forgot" on the interior of the planet, complete with internal sun. In between the two, there's a mantle of miles-deep rock, filled with caverns that make up a more "conventional" Underdark, complete with a resident race of cavern-dwelling outcast elves.
• In Eberron, the Underdark is replaced by "Khyber," the Dragon Below. It's similar enough to bog standard Underdarks, except that there's there's a higher population of Rakshasas present, trying to free their Rajah overlords. Also Night Hags. The deeper you get, the less reality makes sense.
• In Pathfinder, the Darklands are a clone of Faerun's Underdark, based in part on the tiny bits of Underdark fluff that leaked into 3e's stable of Open Game Content. One of the more unique aspects of the Darklands is that its bottom realm, Orv, is composed of a bunch of large "Vaults" constructed by an unknown precursor race that are reasonably interconnected, but hard to get to from the middle layer, existing to scratch your party's itch for pulpy "lost world" style adventures.
• Lamentations of the Flame Princess features the Veins of the Earth. Atomic bees, fossilized vampires, cosmic angler fish that use poorly written bad guys as bait, delicious fungi men, living trains festooned in weapons... the list goes on for the strange things to find down there. It also has its own take on Drow (crazy), Duergar (they think in straight lines), Derro (schizophrenic), and others. It's not a pleasant place to be.
• Dragon Magazine #267 devotes an extensive article to examining four alternate versions of the Underdark; one in which the Duergar have taken over; one in which the Underdark is the bastion of the "good" races against a ruined surface; one in which the Underdark is mostly flooded and thusly dominated by aquatic races; and one in which the Underdark basically is a gateway to the Far Realm.