Just before you descend, you watch as the Ruin Sage draws a blade and cuts the two prisoners free. He drags them out of the gate behind his Anzu, their arms still in their restraints.
The underground chamber is fairly large, perhaps 50 feet in length and half that in width. The first Black Skulls in have lit oil lamps, bathing the damp space in an orange gloom. The walls, floor, and ceiling are all made of moldering cut stone, with more recent timbers bracing it. Everything is leaking, but most of the water runs down to the northeast corner, where the water flows out of a large crack in the bottom of the wall and off into an unknown space. This crack is just big enough for a person to crawl through, and the water cascades into the dark beyond.
Further north is a passage guarded by veteran Black Skulls leading to "the stash." You can make out smaller chambers that appear to be used as storage rooms. They contain a smattering of barrels and crates, containing the week's haul of valuable plants and herbs.
[ +- ] Map of the Black Skull Cellars

Foreman Tunny
You would think Tunny was having the time of his life with this hurricane barreling down. He barks orders and gets these scoundrels and brigands to efficiently set up a sort of camp below. Space is tight crowded, with two dozen odd men below (plus the dogs) but not unbearable.
"This was the Black Skull's first base!" he calls out, as if a history lesson might brighten the men's spirits.
"Me and Dario cleared this place out years ago. That pit there'll do for fresh water if you need it, and you can piss and shit in the outgoing river there. Except for you, Wilmot! You'll have to shit out in the storm, not even a beastman can bear the smell of your ass!"
The Black Skulls have a good laugh at Wilmot's expense.
The dogs stay close to Freki, and since they're not yelping, Tunny doesn't seem to mind. Saffron and Samir also stay close to your group.