The original book had this totally weird set up where the entrance to Jzadirune was a secret door in Keygan Ghelves’ shop, a door that he didn’t even know about. I know that gnomes are weird, otherworldy creatures, but even they don’t build a major underground settlement and then have the only way to get in and out be through a secret door in some random dude’s basement. I also think that Keygan might have noticed hundreds of gnomes and dwarves passing through his shop every day while Jzadirune and the Malachite Fortress were still living communities.
I changed things so that Jzadirune used to have a normal gated entrance, but the city of Cauldron collapsed it after the underground settlements failed, as a precaution against Underdark monsters. The creepers used some of the old gnomish digging machines to bore a hole towards the surface, and ended up connecting the Cauldron’s sewer system.
The captive leads the party down through the crude tunnel and into a large foyer. There are no visible light sources, but the room is somehow lit with a dim, ambient light. There are doors to the west and the south. Belben is dispatched to check out the western entrance, with order to just poke his head around the door frame and not actually the leave the room. The rest of the party checks out the southern door, which is partially closed and seems to be stuck. Jax is able to slip through (Escape Artist check), and then all hell breaks loose.
The tall ones (aka skulks) had been watching the party through peepholes in the walls, and attacks while the party is spread out and divided. A huge battle breaks out, with skulks using their tunnel network to attack from all angles. The jammed door ends up being a huge problem as characters keep needed to cross it to respond to the changing tactical situation, but the only person who can reliably slip through it without using a whole round to do so is Jax.
In the end, the party successfully (but not silently) fists the skulks. Their captive is gone, having used the chaos to escape back up into the sewer tunnels. The party is ragged and wounded, but is now free to explore the immediate area without organized opposition.
The other organized threat in Jzadirune are the creepers, but they always regarded the skulks as unwelcome invaders and were happy to sit the battle out and wait to see what the Fists would do.