Ungoliant

One of the most terrifying evil spirits in this universe, she has the form of a giant spider and may as well be the personification of consumption greed and oblivion itself

Incarnation into Arda
Ungoliant was once one of the Miar. 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 hideos 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 Morguth returned to her lair with a proposition.

Morguth asked her to come with him to Valinor, to destroy it's two most important trees (thus plunging the Valor into chaos). At first, Ungoliant was fearfull, but when Morguth 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 Morguth, that enabled them to slip into Valinor unnoticed. When they reached the two Trees, Morguth stabbed them each of them to the core. But Ungoliant, who had spent years without sustinance, immediatly 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 Morguth. But Ungoliant suffered for this mistake. From that point on, she had a hunger that could not be satisfied.

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

Morguth's Balrogs 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.