Ungoliant

1. Also known as the big U and also as Gloomweaver
2. Little is known of her nature, other than that she is from "before the world".
3. She is able to spin impenetrable darkness
4. She gave birth to Giant Spiders, including the character Shelob
5. She went into hiding her hunger was such that she would mate with other spiders only to devour them later, with her offspring used as food once fully grown