Eldrad

Eldrad is a Kastrian overlord from over 150 million years ago. He was a great scientist who was able to create a barrier to protect Kastria from the solar winds and created new silicon-based bodies to deal with the environment. After this, though, he planned to try to take over the universe with his Kastrian forces, which caused the Kastrians to revolt. These Kastrians, led by King Rokon, were barely able to disintegrate Eldrad. One of his hands survived and somehow made it to Earth.

Eldrad's damaged and petrified hand was stuck in England for millions of years until the quarry it was in was blown up. Eldrad's hand was found by Sarah Jane Smith who was caught in the blast. Eldrad's ring then took control of her mind, causing her to go to the nearest Nuclear Reactor Station. It was also able to influence Dr. Carter and a man called Driscoll. Though the Doctor was able to save Sarah from Eldrad's powers, Eldrad was able to make Driscoll take his hand to the Nuclear Reactor. Using this radioactivity, Eldrad was able to regenerate.

Female Form
Initially after being in the Nuclear Reactor, Eldrad regenerated into a female form (with an appearance somewhat reminiscent of Jabe, though instead made of rock). This form was apparently based off Sarah, since she was the first person to touch the hand.

Eldrad claimed she was falsely destroyed after a war between two other alien races and asks to be brought back to Kastria. The Doctor agrees to take Eldrad to Kastria, but he refuses to go back to the past. Eldrad agrees and leads the TARDIS to Kastria. While in the abandoned Kastrian base, Eldrad is poisoned by an acid harpoon, which starts to break down her system. The only cure is the Regeneration Chamber deep in the base.

True Form
Using the Regeneration Chamber, Eldrad is turned into his true form. The history of the Kastrians is then revealed, as well as Eldrad's part in it. Eldrad plans to begin his conquest again, but learns of the demise of the Kastrians. While Eldrad is distracted, the Doctor is able to steal Eldrad's ring and escapes. While chasing The Doctor and Sarah Jane Smith, Eldrad falls into a deep pit and is destroyed for ever.