Eldrad's development of the spatial barriers allowed the Kastrians to live on the surface but, when he destroyed them, he forced his people back underground. Rather than face what King Rokon described as "a miserable subterranean existence", the Kastrian people "chose final oblivion".
One hundred and fifty million years later, Eldrad returned to Kastria in a female form and went into the thermal chambers. After triggering a booby trap, left in anticipation of his return, and becoming a victim of a specialised acid, Eldrad had the Fourth Doctor and Sarah Jane Smith take her to the regenerator chamber. They jointly supported Eldrad on their way, with the Doctor carrying her over an abyss.
Once he was revitalised and returned to his "true" male form, he searched the chambers for Rokon and found his body. Finding the race bank empty, the Kastrians having destroyed its contents, Eldrad decided to return to Earth and rule over the human race. He pursued the Doctor and Sarah through the thermal chambers. They positioned themselves either side of the abyss and used the Doctor's scarf to trip Eldrad, who plummeted into the depths. (TV: The Hand of Fear)
Eldrad managed to climb out and, in the thermal chambers, built himself a throne room. Sitting in solitude, he went mad. In 2013, Charlie Gibbs travelled with the Fifth Doctor, Tegan Jovanka and Vislor Turlough to the chambers, landing the Doctor's TARDIS there. Through Gibbs and the Watcher another Eldrad was restored who attempted to overthrow the existing one. The dust that covered the floor of the chambers — the remains of dead Kastrians — was used a conductive material by this Eldrad to channel power from the TARDIS.
The Eldrad born from the hand killed that of the Watcher using a dart of the acid he developed. Mulkris, a Kastrian executioner, was reborn from her genetic ring and killed the Eldrad restored from the hand with the same dart of acid. (AUDIO: Eldrad Must Die!)