Korthalos

Korthalos is the domain of the Black Sovereign. Korthalos is a dreary city divided between above ground and underground sections. For half of the day the city is shed in the light of a moon and the other half complete darkness. The city is divided into nine districts, six underground and three above. The population is composed of drow, devils, tieflings, devas, and numerous goblinoid, kobold, and lesser denizens serving as slaves. The city above ground is opulent and regal, holding the highest levels of society; only drow, devils, and devas are allowed to reside on the upper layer. The architecture exhibits the grandeur of society and civilization; the grandest of buildings being the cathedrals dedicated to the Sovereign The surface is divided between the three most senior lords in service to the Black Sovereign. The surface city encompasses the commercial, diplomatic, and bureaucratic elements; while, the underground houses the martial and industrial capacities. In stark contrast the underdark city's spartan and austere settings are home to the vast armies of the Sovereign. The remaining six lords under guide the underdark, each having a level, either controlling mining operations, farming, production, and/or warriors. The vast majority of the slaves reside down there working in the mines or as cannon fodder.

more to come

You asked me a bit ago to put stuff we discussed on here so neither of us would forget it. Here's what I remember off the top of my head.

The three most senior lords (all currently nameless) to the Black Sovereign (those ruling the upper three districts) are:
The Drow Patriarch - the drow district's denizens find themselves in a meritocracy with an aristocratic ruling class, though breaking into the aristocracy is as simple (haha) as sufficiently pleasing the Sovereign...or taking the position by force or wit
The Deva Caliph - pure meritocracy. The Church rewards deeds done in service to the Sovereign with power, status, and wealth.
The Deva Lord (the lordship of the third district has relatively recently been seized from the former devil lord, causing a bit of a stir in the city) - pure caste system. Not really fleshed out, but certain citizens rank above or below certain others, no matter the deeds attributed to them. Although, this system is breaking down somewhat under the influence of the Deva Lord

The districts below are ruled, two by devils, two by drow, and one each by two other races not of the major races found in the upper districts. Tieflings and humans were proposed.

Various forms of movement restrictions could exist, from drow (or drow and deva) being the only race that can move about the city unrestricted (making them ideal secret police) or perhaps only making worshipers of the Sovereign (effectively eliminating many of the slaves and fairly certainly all of the devils) able to move freely about the city.

How does religion work, btw? I was thinking about this earlier, and I was curious if the Sovereign would allow open worship of other deities in his patron city. Do the Knights of the Nine have any influence there at all (I highly doubt they have much if any). Is worship of any other god forbidden, or is worship of the Nine okay, but worship of the Sovereign incentivized with status/rights/movement and the like? Or are certain gods (say the Dragon, the Master, the Shroud) okay to worship but others (like the Swordsman and the Vanguard) not so?

Also, how is the city ruled exactly? Do the three lords (or Nine Lords) get together and vote on things? That doesn't seem terribly like you, and at any rate would make for a terribly unstable system, since I'm sure they would be at each other's throats constantly. Is there some higher power still they they defer to? Or is one of the a First Among Equals...? Actually, I believe we did decide on the Caliph being the First Among equals.

The stability of the city might be served by ensuring that all the races are fairly well interspersed. That is, drow deva and devils live in all the above-ground districts, not just the one ruled by their particular lord.