Spider Silk is a Loot, obtained by completing the missions "The Way of the Timid", "The Art of Gastronomy", "Summons", and "Gripped by Fear", or stolen from Ahriman (loot 4). It is required to create the Ayvuir Red, Masamune, Terre Rod, Perseus Bow, and Germinas Boots.
The Spider Silk and Spider Web are special Accessories that increase EX Revenge Duration by 5% and 10%, respectively. The Spider Silk can be traded for an Attractorb, Moonflow x3, Fal'Cie Husk x3 and 3,500 gil, while the Spider Web can be traded for a Spider Silk, Time Warp x3, Tome of Mystery x2, and 3,500 gil.
A third accessory, Spider's Bane, is even stronger, increasing EX Revenge Duration by 15%. It costs 3,500 gil to trade and needs a Spider Silk, Voidshard x3, and Tome of Mystery x2.
Spider silk is a protein fiber spun by spiders. Spiders use their silk to make webs or other structures, which function as nets to catch other animals, or as nests or cocoons for protection for their offspring. They can also suspend themselves using their silk.