... an evil thing in spider form... ...the last child of Ungoliant to trouble the unhappy world"
From Wikipedia: http://en.wikipedia.org/wiki/Shelob
...But still she was there, who was there before Sauron, and before the first stone of Barad-dûr; and she served none but herself, drinking the blood of Elves and Men, bloated and grown fat with endless brooding on her feasts, weaving webs of shadow; for all living things were her food, and her vomit darkness."...
...She occupied Torech Ungol beneath Cirith Ungol ("Pass of the Spider"). Also called the "Spider of Darkness," Shelob's brood (upon whom she would often feed) and descendants include the Giant Spiders who captured and were defeated by Bilbo Baggins's Dwarf allies in Mirkwood in The Hobbit.