Although the sewer ladder leads to the Irrigation Tunnel, we’ll need to head to the Abyssal Depths. The door to it is found just northwest from where we start our journey. As you go through this door, and enter the new area, you’ll spot the Emperor Leovic group boss room. We don’t want to go there, but to the room just west of it. You’ll find what you’re looking for in the southwest part of this room.
The portal light mentioned in the name might be the glow of a nearby Daedric Portal, or the light coming from the ceiling. There are some laundry ropes in this area. It seems that the inhabitants of these tunnels really like to keep themselves clean and tidy. It doesn’t feel like the best place to dry your clothes, but it is far better than the previous one.