This tool is obtained through killing Sicarii in the Halloween 2008 Quest. Once she is defeated it appears in your backpack. Double-clicking it informs you that you need 10 Gossamers to complete a web. Those can be obtained by looting Death Spiders, Drones and sometimes on Sicarii. They are rather difficult to collect. When you have them it creates A Spiderweb Deed. The Spinneret is destroyed in the process.