With these (aiming for bi-weekly Saturdays) articles we are collecting adventure ideas to use the local cult of the Children of the Rat with their founder Baltram Gorbas in the small city of Honeyngton.
The cult has infested the sewer system and uses it as base of operations as well as a way to travel unseen from one part of the city to another:
with the thicker blue lines being tunnels with pathways at the side of the runnel while the thinner lines are narrower tunnels not even tall enough for most humans to walk upright, though dwarves could:
There are also small diameter pipes connecting individual houses to the system that would just fit rats (or people taking the form of rats or similar sized animals).
The larger blue spots are connecting caverns like this one next to the ruined Praios temple (T6):
Some say, the fire that destroyed the temple was actually instigated by the cult. In any case the cult benefited by building their sanctuary in the cellar of the temple underneath the ruins. The cult spent some resources installing a hidden door in the sewer cavern [Perception (Search) -4, QL 2 needed to detect not just the door but also the opening mechanism] and swarms of rats seem to defend the sanctuary when no cultists are there. Only those knowing or suspecting of the cult's purpose know about this place which includes Baltram, Morena and Geppert.