The planet was constantly ravaged by solar winds. Eldrad created spatial barriers to allow Kastrians to live on the surface. He also developed silicon-based bodies for the species' physical needs and provided his people with inexhaustible power by tapping into the core of the planet as a source. Having done all this, Eldrad regarded himself as the "creator of Kastria".
Eldrad wanted to the Kastrians to conquer other worlds, but the king, Rokon denied him this. In retaliation, Eldrad destroyed the barriers. He was executed for his treason, but the planet was ravaged by solar winds, leaving it barren and cold. Forced to live a subterranean life in the thermal chambers beneath the planet's surface, the Kastrians, in the words of Rokon, "chose final oblivion". They also destroyed the race banks to prevent Eldrad from raising an army if he returned.
When he did so, he was dismayed to find himself without subjects and so decided to have the Fourth Doctor return him to Earth where he could rule over the humans. Pursuing the Time Lord and Sarah Jane Smith, he was tripped by the pair using the Doctor's scarf and fell into an abyss. (TV: The Hand of Fear)
Eldrad survived and climbed out of the abyss where he created a throne room for himself. In his solitude, he became mad, talking to dust that covered the ground in the subterranean Kastrian caves, the remains of dead Kastrians.
The Fifth Doctor came to Kastria with Vislor Turlough, Tegan Jovanka and Charlie Gibbs, the last being under the control of Eldrad through another remnant of his anatomy - an eye. After restoring itself from the eye, the new Eldrad was killed by the old using a dart of acid. Mulkris, an executioner, tasked with gathering fragments of Eldrad, killed the old using the same dart. (AUDIO: Eldrad Must Die!)