The Great One (Planet of the Spiders)

The Great One was the matriarch of the Eight Legs who sought the crystal which the Doctor had stolen and taken to 20th century England.

Physically gigantic, the Great One lived in a cavern made of blue Metebelis crystals. She believed that with the last great crystal placed above her head, she would have infinite power and knowledge. The radiation of the crystal, instead, destroyed her.