Some stupid soap dwarf falls into a secretive mood and takes over a craftsman workshop. I have to slaughter a few animals to meet his sick fantasies, but he starts requesting some kind of cloth I don't have. I trade for Fiber cloth. Nope. I trade for yarn. Nope. It's at this point I realize that the jerk must want some silk (that the caravans don't happen to be carrying), so I begin an expedition into the heart of a dangerous cavern in order to sate his creative spirit. As I venture close to the cavern, a horrifying roar from its dark depths signals the approach of the Forgotten Beast. As the alternative to spelunking would be allowing my soapy friend to go insane and starve to death, I opt to throw caution to the wind and proceed anyway. I spot some spider silk near the edge of the cavern, and plan to quickly break through the wall, steal it away, and seal up the passage before the beast can discover my dwarves. The beast no where to be found, my miner breaks through the single layer of rock.
Suddenly, from out of no where, the beast charges with ferocious speed into the newly opened passage (seriously, the second I opened the passage he just zoomed in). I send my ramshackle militia to their doom in order to buy some time while my masons seal the passage behind them. The Forgotten Beast then finds its first victim, the blissful weaver I sent to retrieve the silk. I quickly deputize her to give my militia time to close the distance. She manages to get a few decent hits in before being turned to rot by the beast's venom. My militia arrives just as she dies, and sets upon the creature. The beast immediately kills poor Speardwarf #3, and incapacitates another two before sending a third flying across the room. Miraculously, it cannot not withstand the might of my entire militia, and soon dies under their onslaught, ultimately only taking three and half dwarves with it. My fort and the caverns free of its terrifying presence, I send another weaver to retrieve and weave the silk.
Nope.
Apparently, Adamantine can be woven into cloth, and dwarves in fey moods will not work with anything but once you discover it. My dwarf winds up going insane, and I'm forced to lock him in his room until he starves to death. At least, he was just a stupid soap dwarf. On an optimistic note, the dwarf that lost her left arm to rot in the battle for the Spider silk is still able to cut down trees just fine.