Going down into the warm room once again, the party keeps a close watch for demons.
At first, there are none. The first slide down leads to a broad chamber, smooth in the middle from many feet, with the sides still showing grit from native stone. The demons have been down here a long time, and in great numbers. In one corner, there are some drops. A demon died there, dropping 1x Brown Demonhide. You loot it, and move on.
The second chamber splits. The smooth path splits in two, one going through a more narrow tunnel, distant light visible through tiny holes in the wall. The main path continues downwards. After a few turns, small holes open up to a large sulfurous chamber, like tiny windows of orange light. You're not nearly deep enough for magma, but who knows what odd lights demons use in their great pits?
This turn ends in some long-unused machinery, now a pile of rusted iron slag. Nothing here for a party of high level adventurers. It splits again here, with a smoothed hole going into a dark room full of uneven shapes.. a peek reveals them to be broken shelving and piles of random bits. Probably something of value, but it's a much more poorly curated junkroom than Uncle's.
Further down..
You come across some proper demons. This room has a spring and a pool of water, all colourful from who knows how many mineral salts. There are another three or four passages here, and peering at the edge, no less than a dozen non-humanoid quadrupedal demons, sitting on the rough ground, staring at each other. One is up, with its head down, looking into the depths. Another is scraching itself like a dog.