Eldrad

Eldrad was a Kastrian overlord from over 150 million years ago. He was a brilliant scientist who created a barrier to protect Kastria from the solar winds and new silicon-based bodies to deal with the environment. He then planned to take a more offensive stance with his Kastrian forces, which caused the Kastrians to revolt. These Kastrians, led by King Rokon were able to capture him and planned to annihilate him. Though he was almost destroyed, one of his hands and his ring 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. Sarah Jane Smith was caught in the blast and found Eldrad's hand. Eldrad's ring then took control of her mind, forcing her to go to the Nunton Experimental Complex. Though the Doctor was able to save Sarah from Eldrad's powers, the ring was also able to influence Dr. Carter and Driscoll, who also tried to help Eldrad regenerate. Eldrad was able to make Driscoll take his hand into the core of the Nuclear reactor. Using this radiation, Eldrad was able to completely regenerate.

Female Form
After he regenerated, Eldrad turned into a female form. This form was based off Sarah, since the ring imprinted off her.

Eldrad claimed she was overthrown after a war between the Kastrians and an unknown race and asked to be brought back to Kastria. The Doctor agreed to take Eldrad to Kastria, but he refuses to go back to the past (which would disobey the Laws of Time). Eldrad agrees and leads the TARDIS to Kastria. While in the Kastrian base, Eldrad is poisoned by an acid harpoon, which starts to break down her system. The Doctor and Sarah are able to bring Eldrad to the Regeneration Chamber just before her structure breaks down.

True Form
After using the Regeneration Chamber, Eldrad was restored to his true form. He planned to take back Kastria and then the universe until he learnt of the eventual fate of the Kastrians. His original plans foiled, Eldrad decided to take over Earth and enslave the Humans, using the TARDIS to get back. The Doctor was able to steal Eldrad's ring and tried to escape with it. While chasing The Doctor and Sarah, Eldrad was tripped by the Doctor and Sarah. The two pulled on the ends of the Doctors scarf this tripped Eldrad who fell into a deep pit. This presumably killed him although the Doctor expressed doubt about Eldrad's demise. (DW: The Hand of Fear)