Neutral people who aren't invincible needed a safe haven, so they created The Underworld Amphlitheater, then decided it was a dumb name so they called it Underdark instead.
I already fucking talked about htis, but if you want to hear more then fine...jeez
Underdark started as a safe haven for Neutrals, those who do not take a side in the fight of good versus evil. Located underground somewhere in the european continent, they stayed there when things got rough on the surface.
COME ON I SAID IT
Over time many awesomes both good and evil, government organizations, and bad people found the place and wrecked it up. But the Neutrals weren't entirely powerless so they just beat the crap out of whomever started causing trouble. From then on anyone who entered Underdark was forced into peaceful actions or else they would get the crap beaten out of them.
They needed money and things to do, so they set up black markets, spy-nets, gladiatorial combat, game shows, enthusiast conventions, and other enterprises. After that they received many clientel needing information brokering, appraisals, or a cold beer. Call it an underground hub if you will.
This is the domain of anyone truly neutral, they do not take sides and they do not interfere in anyone's affairs. Whie this is a safe haven for them, it may not be for visitors. They have few rules which are:
1) Do not attempt to bring a neutral harm in any form (physical assault, pilfering, mental anguish, bad pick-up lines)
2) Do not attempt to recruit a neutral to your cause (kill all humans, save the puppies, etc)
3) Do not trash the place, we'd like to keep it nice (litering, vandalism, noise pollution)
4) Curb your pets (pick up their poop, dumbass)
5) Do not use the Underdark as base of operations for your cause (Guilds are different and do not count)
6) Try one of our various flavors of ice cream (Dr Tasty is banned)
7) No washing dishes in the bathroom sinks
8) Keep any quarrels above power level mediocre in the pit
9) Do not cause damage to Underdark (this does not include Non-Neutral facilities)
10) Be pleasant to your elders
Everything else is fair game and very dangerous at times. If one is not prepared for the danger of Underdark, they may find themselves murdered or worse. Valuable goods can and will be stolen if not properly guarded. You will walk through clouds of farts and vomit smells.
If a quarrel errupts and is more powerful than a mere fistfight, it is mandatory to go to the Pit to battle, as the pit is a reinforced gladatorial ring for fights between powerful beings. If two or more awesomes do not adhere to this rule then they are promptly either teleported to a random location on earth or blown to smithereens (usually the latter.)
Underdark is a very nice place, hence the introduction of the no trashing rule. There are nice parks and gardens, the streets are shiney and you can find a trash recepticle on every block, which are maintaned by the freelance garbage corps.
Littered (or scattered, don't kill me) throughout Underdark are seedy corporations and guilds in tight passageways and dark buildings, severely contrasting the rest of the place. It looks like a mix of bartertown and modern san francisco.
Various heads of guilds set up shop here, wanting to do business from a neutral and safe point. If you are a member of a thieves guild you would go here after your initiation, if you want to sign up for a fighting company you would go here to prove your worth and to obtain gear, etc. The place is vast and heavily enforced by neutral awesomes.
It is not uncommon for two nemesis to appear here without fighting and it is not uncommon to see executions being performed. Whatever it is, the only thing you won't see is the Neutrals taking sides. Neutrals run the show here, but they are not available for recruitment by any organization. However, sometimes one or two of those guys will have a change of heart and acclimate back into the surface world.
The Revengerists have at times used this as a place to aqcuire various forbidden goods, machines, and personel.