Underdark

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

The Underdark is a location in Dungeons & dragons that originated out of Greyhawk and gained popularity due to the Forgotten Realms. Specifically, the Underdark refers to the massive cave system that plunges into the depths of the world - unlike the Hollow World of Mystara (or any homebrew setting using the Hollow Earth form of Setting Aesthetics), these are "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 Hook Horror to sapient races. The most iconic races, however, would probably be:

• Drow
• Duergar
• Svirfneblin
• Illithids
• Kuo-toa

Faerun's Underdark

The Underdark first appeared in Greyhawk, as part of the adventure module series "Against The Giants", although it would not be properly explored until its successor series, "Descent into the Depths of the Earth". This introduced us to the drow city of Erelhei-Cinlu.

When Forgotten Realms took off as TSR's baby instead of Greyhawk, the Underdark was promptly migrated over to bedevil the denizens of Faerun. It would become an iconic aspect of 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 has ever reached this place, not even magical light (which doesn't work down there- Hah! take that Wizards!). 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

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 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 Deeps

This covers every other part of the Underdark as lies beneath the World.

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.

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.

Other Settings

• 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.