The Great One was a gigantic spider who reigned as the goddess of the Eight Legs. She sought to rule the universe and needed only the crystal which the Third Doctor had stolen, taken to 20th century Wales and given it to his companion Jo Grant as a wedding present.
- You may be looking for the deity.
The Eight Legs obeyed the Queen Spider, who also obeyed the Great One. Being physically a giant, the Great One lived in a cavern made of blue Metebelis crystals. With her psychic power she could read the Doctor's mind and discovered how fearful she made him. Obtaining the Doctor's crystal, which was the last perfect crystal she needed, she completed a web of the crystals which lay above her head and she was able to unlock her mind.
Infinite power and knowledge were fed into her mind, but since there was no way to vent it, radiation from the crystals built up in the web and the crystals burned her mind, destroying her. (TV: Planet of the Spiders)
Another Eight Legs Queen later declared herself to be the new Great One. It was killed when the Headhunter tricked it into falling for its own hypnotic spell. (AUDIO: Worldwide Web)