Ungoliant

In J.R.R. Tolkien's Middle-Earth, Ungoliant was a great spidery demon, which worked with Melkor to break and corrupt the world, until they had a falling out over the disposition of a few Silmarils looted from Valinor. Ungoliant had several lesser offspring, the best known of which is probably Shelob; I have also heard that the giant spiders of Mirkwood are ultimately descended from Ungoliant as well (via Shelob). Those spiders, while large, were mere mites compared to their progenitors.

There is an amusing relationship between the Bored of the Rings characters, Sorehed and Schlob, which I think is really more a cognate of Melkor and Ungoliant's special relationship, than Sauron and Shelob's acquaintance.

Ungoliant Gloomweaver* entered the world clad in darkness, perhaps when Morgoth first looked upon it with a darkened mind, and there she took the form of a many-eyed spider. She hated everything, and most of everything she hated light, yet light was what she most longed for. Trapped in her own webs of darkness Morgoth found her and made a pact with her: If she helped him strike against the Valar, he would give her all she desired.

The great spider wove a cloak of Unlight around them so they could not be seen, and they went to Valinor clad in secret. There Morgoth stabbed the two trees of light and let Ungoliant suck their light out and fill them with poison. From all this light she grew larger and darker than she had ever been, so that even Morgoth was scared of her. Hidden in a cloud of dark smoke they fled north, and Ungoliant demanded what he had promised her. She wanted all the jewels he had stolen in Valinor. He let her devour them all, except three, the most magnificent of them all: The Silmarils. Refused those gems of light Ungoliant assailed the Dark Lord with all her might, but he called out and his serving Balrogs came to his rescue.

The great spider left the north and found a refuge in Nan Dungortheb, the Valley of Dreadful Death, where she mated with other monsters of spider form. Her offspring would trouble Middle-Earth long after, and most of all Shelob, her last descendant. Ungoliant went south from the valley and disappeared from the tales, but it is said that she met her end when at last she devoured herself.

(adapted from The Silmarillion)

Tolkien's arachnophobia is not hard to spot in his works; in each of his major stories great evil is done by large spiders. Ungoliant kills light in the Silmarillion, Shelob attacks in The Lord of the Rings, and the spiders of Mirkwood nearly kill the Hobbit and all his company. Oh well, I suppose describing your nightmarish fear in the detail works as therapy for some.

* According to the Encyclopedia of Arda, Ungoliant's name stems from ungwë liante, 'gloom spider', but 'gloomweaver' is Tolkien's preferred rendering. It does sound better, doesn't it?