Alastriona and Butler are poking up the north hall down the stairs and scoping out a sleeping quarters of sorts around the bend. Currently holding position.
Going back to the central hub, there are also some other locations to investigate. There are 2 arrow slits in the hub. You've found Eins in the Northwest one, but there is also a slit in the southeast side of the hub (barely shows on the map, sorry). In addition, to the south of the hub you made out a side tunnel veering off of the main sewer.