Go southeast from the Lambent Passage entrance. You can find the General Zamachar group boss room in the southern part of this area. Head north from his room, into the room with the high ceiling and a large central stone pillar. There’s another sewer drain in the northern part of the room, in the middle of a small pool.
It is just on top of the drain, slightly slipping into it. We are lucky to find it here – like the name says, it could slip down the drain soon, in a couple of hundreds of years. Beside the patrolling enemy, the others found nearby are fairly easy to kill. This area is also described as Nocere Oblitus, where the cruel wards of Zamachar nurture only the strongest foes.