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. Eldrad was then annihilated, though one of his hands with his ring 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 then found by Sarah Jane Smith. Eldrad's ring then took control of her mind, causing her to go to the nearest Nuclear Reactor Station. Though the Doctor was able to save her, Eldrad was then able to control Driscoll and his hand was brought 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 was apparently based off Sarah Jane Smith, 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 only to the present Kastria. She agrees and leads the TARDIS to Kastria. While in the Kastrian base, Eldrad is poisoned by acid, 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, along with what happened next. 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.