Gloomhaven's water supply appears to have been poisoned. You've been asked to investigate the source of the poison. Unfortunately, this means trudging through the sewers. Yuck.
You begrudgingly hopped into the sewers to investigate the poison. You slew plethora of snakes, ooze, and Vermlings. The Vermlings had made camp around a cisterns that appeared to be the source of the poison, but upon further investigation, none of the dead Vermlings had any poisons at all. There is a passage at the back of the room that leads deeper or, if you had a way to breath underwater, you could enter the cistern directly.
You've found a way to the source of the poison. It's time to purify the water.
The water pumps were teeming with unholy creatures and a writhing black corruption. By the time the fight is over, the walls and floor are a mess of black ooze, but you managed to get it all out of the pumps. None of it is pulsating anymore either, which is a step in the right direction. A simple mop can clean the remnants, now that the dark presence has been removed.