For all the aquaducts coming into the city, there munt be a place for the water to leave. Thus, many years ago, the Blackwater Tunnels were commissioned to carry sewage away from the city buildings. Consisting of a great hive of brick-lined tunnels great sloping away from the city, the Blackwater tunnels serve as an excellent path in and out for less desireable elements, and though the Guard does their best to hold choke points, the maze is simply too great for them to control in entiretly. Theives and assassins, however, are not the worst of their trouble, nor is plugged sewage.
Of late, a black cult has found a home amoung the sewers, worshipping a horrible lord of death, disease, and terror. They have rebuilt several areas of the Blackwater, creating a well hidden, easily defended fortress beneath the city proper. Worse, with the comfort the security of this installation gives them, they have redoubled their efforts to spread plague and fear amoung the city above.
More, strange things that no one, not ever the cultists, have previously imagined have often appeared within the Blackwater. Strange beings akin to water elementals, yet made of human wastes roam the tunnels freely, feeding upon the refuse of the city above. Roaches and other scavenging insects grow to abnormal size here, to say nothing of the rats.