Eldrad is a Kastrian - a crystalline, silicon-based being. And not just any Kastrian; Eldrad likes to call himself the creator of Kastria. Indeed, this claim is 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 Earth. 150 million years later, Sarah Jane Smith found Eldrad's hand in a quarry.
Eldrad's power is in his ring. Contact with the ring establishes a telepathic connection between Eldrad and the unwitting human, enabling Eldrad to control them. The directive "Eldrad must live" is foremost in the mind of the person Eldrad is controlling, meaning that they will do anything to ensure Eldrad's survival. The ring can also be made to emit a blue light, called the "light of Kastria", which can stun people briefly or at least up to an hour, and destroy objects. The ring also contains Eldrad's genetic code, with which Eldrad can completely reconstitute himself, given suitable radioactive material. Eldrad absorbs radiation to regenerate his body, a system of creating matter from energy. This new body is modelled after the first being into which Eldrad comes in contact, in order to ensure that his new body is in an acceptable form.
Eldrad's eyes glow bright blue when he is reading someone's mind, to determine whether or not they are telling the truth, and also serves as a weapon that can kill people, possibly through vaporization. He can also burn his way through very thick metal. Any weapon that works against silicon life forms will affect Eldrad, particularly a type of acid that neutralizes his molecular bond, although if he has access to a regeneration unit, he will still survive.
Eldrad tried to win the trust of the fourth Doctor and Sarah by telling them that two alien races fought a war on Kastria, destroying the barriers in the process, made puppets of the Kastrian leaders and discredited him. He asks them to help him save Kastria, and they agree. But his real intent was to gain the control of Kastria that he had always wanted, believing that the inhabitants had retreated underground to a miserable subterranean life. When he learned that the population had died rather than endure such a fate, he intended to reconstitute the one hundred million Kastrian subjects that had been stored in the race bank as crystal particles. But, suspecting that Eldrad might do just such a thing, the Kastrians had elected to destroy the race banks as well. Enraged, Eldrad decided that he would rule Earth instead; but the Doctor and Sarah tripped him with the Doctor's scarf, causing him to fall into a deep abyss.
