- "The Eldar knew not whence she came; but some have said that in ages long before she descended from the darkness that lies about Arda, when Melkor first looked down in envy upon the Kingdom of Manwe, and that in the beginning she was one of those that he corrupted to his service."
—Of the Darkening of Valinor, The Silmarillion
Ungoliant is the largest of the giant spiders in the Tolkienverse. She is speculated to be a Maia that sided with Morgoth early, becoming stuck in spider form permanently. However, later events turn her into the stuff of nightmares, and one of the most terrifying things in The Silmarillion.
In Canon
Ungoliant is known for two things. The first is helping Melkor kill the Two Trees of Valinor by draining them of their light, thus plunging Valinor into darkness, killing Finwe, High King of the Noldor, and allowing Melkor to steal the Silmarils. This led to Finwe's son, Fëanor, going into a mad rage over the loss of his precious Silmarils, and thus leading most of Valinor's Elven population to Middle-earth, where the War of the Jewels was fought for the next few millenia until Melkor's downfall. Sauron rose to power, mind brimming with ideas of achieving world domination with the use of really powerful jewelry, and the rest, as they say, is history.
The second thing Ungoliant is known for is for spawning the race of giant spiders with the help of other rogue Maia (whom she then ate). Most of her descendants live in Mirkwood, hidden in the darkness of the forest. Her daughter Shelob, however, guards the pass of Cirith Ungol on the border of Mordor.
Ungoliant seemed to enjoy devouring light, as she drained it from the Trees and some non-Silmaril jewels Melkor had stolen when they attacked Valinor. Apparently, the trait wasn't passed on to her children, as Shelob definitely hated light when Samwise Gamgee shined the Phial of Galadriel into her eyes in The Two Towers.