Ungoliant

Ungoliant was one of the most terrifying evil spirits in The Lord of the Rings universe, she has the form of a giant spider and she may as well be the personification of consumption, greed and oblivion itself.

Incarnation into Arda
Ungoliant was once one of the Maiar. But she, along with many others, was suduced and corrupted by Melkor. When Melkor turned his followers into demons, such as Balrogs, Ungoliant took the form of a hideous giant spider. Enraged, she disowned her master and fled to the forsaken wrelm of Arda, to take refuge from the Valar and the hunters of Orome. There, she hid in a ravine beneath a fotress of black webs, and spent years of lust and hunger.

Union of the Dark
Ungoliant was once a partner of Morgoth. But she abondoned him, and became her own mistress. She did, however, rejoin the dark side when Morgoth returned to her lair with a proposition.

Morgoth asked her to come with him to Valinor, to destroy its two most important trees (thus plunging the Valor into chaos). At first, Ungoliant was fearful, but when Morgoth said; "Do as I bid; and if thy hunger still when all is done, then I will give thee whatsoever thy lust may demand.", she agreed to help him.

Ungoliant wove a shadow around herself and Morgoth, that enabled them to slip into Valinor unnoticed. When they reached the two Trees, Morgoth stabbed them each of them to the core. But Ungoliant, who had spent years without sustinance, immediately sucked up all the magical sap. She then grew to twice her normal size, but she was still thirsty. She went to a sacred well, and drank it dry, growing so large she even frightened Morgoth. But Ungoliant suffered for this mistake. From that point on, she had a hunger that could not be satisfied.

Morgoth and Ungoliant fled back to Middle-earth, where they escaped from the Valar and the Miar. Morgoth, who had seen enough of Ungoliant, attempted to slip away. But Ungoliant held him back, and demanded that he keep his promise. Grudgingly, Morgoth gave her the gems of Formenos, and she grew larger than ever. She then demanded the Silmarils, but Morgoth refused. Ungoliant attacked Morgoth, and began wrapping him in web.

Morgoth's Balrog attacked her and drove her into the south of Middle-earth, where she met many man sized spiders. She mated with them and gave birth to a more advanced race of immortal giant spiders. Ungoliant remaned in the South, and began eating everything. The giant spiders, including Shelob and the rest of Ungoliant's children fled to escape her hunger. Eventualy, Ungoliant's hunger grew so out of control, that she ate herself to death.

Legacy
Shelob was the greatest of her daughters, whom of which encountered Frodo and Sam on their journey to destroy the One Ring. Shelob was potentially the last of her kind, though it is unknown if she lived or succumbed to the stab she recieved from Sam.