The Drakkenheim sewers were a sophisticated engineering marvel. The system combined underground gravity-based aqueducts and drainage canals with carefully placed locks and floodgates to deliver clean and fresh water to the city population and draw away the massive quantities of sewage and wastewater. Major canals run under the city streets, and can be accessed from the surface by numerous maintenance hatches.
After over a decade of disuse, most tunnels are flooded with contaminated water and many other passages have collapsed. Other sections are now monster lairs. As a result, travelling in the sewers is extraordinarily hazardous and ill-advised. However, characters might consider taking the sewers to locate a route under the city walls, or to gain access to hidden underground dungeons such as the cistern or the Court of Thieves.
Sewer Navigation. Handle sewer navigation abstractly using the same mechanics for navigating the streets above, except characters move at half their normal pace through the twisting tunnels. Sewer passages and access points could be almost anywhere in the city. In addition, after each hour spent in the sewers characters must succeed on a DC 10 Constitution saving throw or gain one level of contamination from contact with sewer water.
Searching the Sewers. Just as characters may search the streets, they might search the sewers for hidden entrances to some adventure sites. Other secret locations may be found in this manner. A DC 20 check result is needed for success when searching the sewer passages.