Eldrad was a Kastrian overlord who was sentenced to death and obliterated.
Biography
A hundred fifty million years before the 20th century, Eldrad was a Kastrian - a crystalline, silicon-based being. And not just any Kastrian, Eldrad liked to call himself the creator of Kastria. Indeed, this claim was not exaggerated. The planet of Kastria was originally a very sterile place, frozen by the solar winds that blew across its surface. Eldrad built spatial barriers to hold out the winds, then built crystal-silicon bodies for the Kastrians, followed by machines that would replenish the earth and atmosphere. He devised a backup system by which the Kastrian domes could draw energy from the core of the planet.
Eldrad's mistake was in deciding that the Kastrians were too unconcerned with power. He felt that the Kastrians should conquer other races to become the masters of the galaxy, and intended to replace Rokon as king in order to accomplish this. The Kastrians, however, were not fond of this course of action, so Eldrad destroyed the spatial barriers in revenge. For this most horrendous crime, Eldrad was sentenced to death and launched into space in an obliteration module, designed to be blown up when the capsule had gone beyond the boundary of the solar system. But lest the power fail before Eldrad had been killed, the Kastrians blew up the module early, with a one-in-three-million chance that part of Eldrad would survive. Against the odds, part of Eldrad did survive - most of his left hand, which plummeted to primordial Earth.
Eldrad's damaged and petrified hand was found in an English quarry after an explosion. Having just arrived on Earth in the TARDIS with the Fourth Doctor, Sarah Jane Smith was caught in the blast and found the hand. Eldrad's ring took control of her mind, forcing her to go to the nearby Nunton Experimental Complex, which had a nuclear reactor.
Though the Doctor saved Sarah from Eldrad's powers, the ring also influenced Dr. Carter and Driscoll, who worked to help Eldrad regenerate. Eldrad made Driscoll take his hand into the core of the reactor. Eldrad employed the reactor's radiation to completely regenerate into a form modelled on Sarah, the ring having taken her imprint.
Eldrad claimed she had been overthrown after a war between her people and another race, and asked to be returned to her homeworld. The Doctor agreed to take Eldrad to Kastria, but refused to go into the past. Once in the Kastrian base, Eldrad was poisoned by an acid harpoon, which started to break down her system. With the help of the Doctor and Sarah, she reached a regeneration chamber before her structure broke down.
Restored to his true Kastrian form by the regeneration chamber, Eldrad planned to take back Kastria and then the universe, until he learnt of the fate of the Kastrians. Led by Rokon, they had chosen suicide for their race rather than risk the small chance of Eldrad returning to lead them. Eldrad decided to return to Earth in the TARDIS and enslave the humans there.
The Doctor stole Eldrad's ring, causing Eldrad to pursue him. Chasing the Doctor, Eldrad tripped over his scarf and into a deep chasm. Though Eldrad seemed to have died, the Doctor expressed doubt about his demise. (TV: The Hand of Fear)
Powers and Abilities
In addition to the abilities of regeneration and metamorphosis granted him by his silicon-based form, Eldrad could use telepathy. His hand could control the minds of humans who had seen the light of his ring. In his female form, she attempted to hypnotize the Doctor. When she did so, her eyes glowed bright blue. (TV: The Hand of Fear)