Outcasts of Might and Magic and Righteous Revenge
An ancient family of vampires, the Fortissas, established Mal around an underground river and grew it to the surface.
The protrudes downward (from the perspective of above ground fleshies), divided by distinct sections of increasing status. Travel between levels is dependent in the exclusivity of the level, and there may or may not be some kind of guard mechanism around the ramp leading down into the ground.
Top: The Gardens
Where most of the fleshies and new arrivals live, and a good deal of passing commerce happens. No coverings for light sensitive undead, but the strong law keeping nature of the Fortissas extends to even here.
B1: Merchant's Landing
Where the vast majority of shops and everyday business happens. Also the home of the lower middle class and where the first few layers of undead residents start mixing in with the fleshies.
Where courthouses, permit offices, police stations, and other civic buildings are interspersed with more established residences, leisure and entertainment zones, and public magical schools. It's like the american suburbs, but underground and more rotting sentient skeletons.
Where we see varying degrees of nobility and noted specialists work their brand of magic. Long unlived undead have preference in the court, if for not other reason than their time advantage towards building a legacy. It is not uncommon for particularly charismatic fleshies to make themselves worthwhile. A spark, though momentary, can bring a welcome light that has the added benefit of not burning cursed flesh.
B4: Fortissa, and its River
Where the ruling family is said to reside. The River is what allowed the Fortissas to start a limited underdark trading venture. This may all be false rumor, as there are no known entrances from B3 to B4. The evidence of the Fortissas as a ruling force are the missives the local executive, legislative, and judiciary powers get passing on the will of the family, and the crack team of undead enforcers working in the name of the Fortissas that keep peace among the undead crowds when the law moves too slowly.