These spiders are gigantic - the size of a horse.
Stygian spiders are highly intelligent and are capable of human speech. They engage in business with those who seek them out and are experienced hagglers. Unlike normal merchants, however, stygian spiders demand extremely strange prices for their spidersilk: dreams, memories, youth, beauty, souls, and the like.
Their diet is thus far unknown.
The areas surrounding stygian spider dwellings are filled with normal-sized spiders seeking out the protection of their large magical cousins.
They possess magical abilities, one of which may be shapeshifting.
In Throne of Glass Society
Of all their unique traits, stygian spiders are probably best known for producing spidersilk. This material is lightweight yet extraordinarily durable, extremely strong and gleams faintly in the sunlight. Unfortunately, obtaining this precious material comes at a high cost: The spiders do not trade their silk for gold, but for much less tangible and more valuable things like dreams, memories, youth, beauty, and souls. Because of the low supply - resulting from the difficulty of acquisition - of spidersilk and its high demand, the market price for it is generally astronomical.