"This giant spider shimmers as its body phases through the several dimensions of physical space."
Found on the Cryptobarrio floors of Bethesda Susa. The first floor of the Cryptobarrios will always have one next to the stairs down.
The spider is constantly phased, meaning it will only attack you when you phase yourself, either by the phase mutation or by stepping into one of its phase-webs.
Its shimmering webs can be used to phase through the eastern wall to reach the stairs to the next level. (although this is dangerous, since phasing in while on a wall tile will kill you).
When exiting the webs it places, they are sometimes harvested for one charge of Phase Silk. The spider will place more webs and so can be farmed for silk, although doing so is tedious and dangerous.