Shelob is a gigantic spider, the 'last living child of Ungoliant the Great'. She lives in a tunnel high above Minas Morgul. Sauron was aware of her, and used her to guard his border. When Gollum brought Frodo to the tunnel, Shelob wrapped Frodo in a cocoon. After that, Sam managed to wound Shelob, and she retreated back into the tunnel.