The Ryth is an order of giant spiders that gained intellegence a long time ago due to the meddlings of an insane wizard.
They are spiders- thus they do not possess thumbs and thus do not build things or use tools.
However, they can dig with great skill with their large mandibles, and their silk can be hardened to form an effective wall.
They are burrowers- carving out tunnels and nests deep inside caverns, abandoned mine shafts and the like, spinning great webs much like their unintellegent cousins.
Each lair is controlled by a Broodmother, a massive spider capable of swallowing a man whole. Laying massive clutches of eggs at a time, the Broodmother spins a great web where she and her children thrive, grow, and wait... for when their lair is breached by some poor miner, the world will be their new home.
Occasionally, the Ryth come into possession of civilized creatures, sometimes when they accidentally breach their lair, and sometimes when they happen to 'fall' in through a sinkhole. These poor men and dwarves and preferably elves are kept in massive cages, forced to breed to feed the bloodthirsty Ryth.
In more technical terms, occasionally during worldgen, a Ryth nest will spawn on a map tile anywhere. An average Ryth nest contains:
One Broodmother Megabeast. Think of it like a giant giant giant mega cave spider of doom.
A whole bunch of large spiders. They function like ants, with various professions assigned at birth, ranging from soliders to workers to thieves capable of tunneling to the surface and stealing children, adults, and cats.
An even BIGGER number of tiny spiders. Not cave spider vermin, but spider hatchlings. Ryth take a long time to mature, and they don't always survive. The tiny spiders aren't dangerous by themselves, but a whole bunch of them will usually end up with the victim being eaten alive, piece by piece.
A few human/dwarf/elf slaves. There will only be one kind, based on what kind of settlement is closer. These are treated as livestock, constantly breeding, and being eaten by the Ryth. By the time worldgen is complete, these prisoners have lost all hope and will serve the spiders without question, with the exeption of the occasional rebellion. Which usually consists of one person trying to escape, and promptly being eaten by something.
The nests would contain a massive number of webs, but visible. These range from the massive web that the broodmother sits on, to solidified webbing forming walls and the like.
There are also glowing crystals/mushrooms everywhere which should provide light when the lighting arc is in. They could be harvested by dwarves to provide easy light sources.
On a scale of 1-10, I rate the difficulty of this encounter a 9.
The Ryth aren't AS bloodthristy as the GCS, but they come close. The caverns are huge, and thus you won't fight too many spiders at once, and the broodmother would be a pain to fight, (seeing as how she can swallow you whole.) but once artifacts have magical properties, a nice !!fire imp bone crown of resist webbing!! should keep you safe. Well, sort of.