Firstly, Necromancers will only resurrect skeletons because I don't understand why a city would be needed otherwise if a hero in the open field can create vampires, liches, and undead. In that case, Necromancers should be nomadic.
Secondly, the Necropolis now consists of three sub-factions: the Cult of Death, the Corpse Eaters, and the Ancient Undead. To avoid a Necromancer having only skeletons, they are forced to gain the support of these factions. They pursue their own political games and have embassies in the Necropolis.
The Cult of Death is a branch of the "sea" cult.
The Corpse Eaters are the result of experiments by ancient dark mages; these are disfigured souls that create bodies for themselves by modifying corpses.
And the Ancient Undead are the guardians of the culture of the old human empire, which was ruled by dark mages. These warriors hunt dragons.