We need to reach the Vile Drainage from Lambent Passage. The door to this area is found as you head toward the center of the sewer. It is the first door you have to pass through in order to continue your journey. As you enter the Vile Drainage, head toward the Bloodmist Slaughterhouse. Within the second room, east of it, is another circular room. Find a flag with a lion painted on a blue background on top of it. Get to the platform with it, next to some wooden boxes. If you’ve pulled some of the enemies before this part, you can just jump on this platform, and they’ll stop the chase.
The “lion” part of the name is clearly not a mystery once you find it, but the cistern part was a bit confusing for a moment. Maybe the circular room reflects something that should be a cistern. I am out of ideas for this one.