The Kastrians were natives of the planet Kastria. They were silicon-based lifeforms, making them near impossible to kill and allowing for their restoration under the right circumstances from single particles.
The scientist Eldrad provided them with high technology. His betrayal, which left Kastria a victim of the solar winds, led the Kastrians to choose suicide rather than be consigned to life underground on their desolated world.
Eldrad was able to restore himself after his hand survived along with a ring containing his genetic code. Sarah Jane Smith was the first human he had contact with, so he adopted her form — becoming female. (TV: The Hand of Fear) Mulkris had her own genetic ring which restored her to life by taping into the power of the Doctor's TARDIS. (AUDIO: Eldrad Must Die!)
Eldrad produced several technological marvels for the Kastrian people. He built the spatial barriers to protect the planet from solar winds, allowing for life on the surface, and rejuvenated the earth and air.
Furthermore, he created the regenerator chamber, which could revitalise Kastrians with irradiation. This was kept in their subterranean dwelling. Eldrad produced an acid that neutralised the molecular bond.
The Kastrians had several forms of spacecraft, including obliteration module, which were used for executions. They would be detonated a set number of spans away from Kastria to ensue that the condemned did not survive.
The Kastrian race bank contained a hundred million crystal particles which could be used in conjunction with the regenerator to create Kastrians. (TV: The Hand of Fear) Genetic rings could also be used to restore Kastrians to life, with their individual genetic codes being retained within. (AUDIO: Eldrad Must Die!)
The Kastrians flourished around 150 million B.C.. During the Millennium War, they attempted an assault on the Mad Mind of Bophemeral, but their fleet was utterly destroyed and their planet was ruined when their computers were turned against them. (PROSE: The Quantum Archangel)
To shelter from the solar winds, Kastrians resided in the thermal chambers, a network of caves beneath the planet's surface. The race bank and regenerator chamber were located there. Eldrad provided the Kastrians with the technology necessary to survive on the surface, in spite of their planet's environment. The spatial barriers and his revitalisation of the earth and air was a great boon. However, he wanted to see galactic conquest pursued by King Rokon, but he refused. Angered by this denial, Eldrad destroyed the barriers, leaving the planet exposed. He was sentenced to death and placed into an obliteration module. Due to fears that the solar winds would cause Outer Dome Six to lose control over the craft, it was detonated at nineteen, rather than twenty-five, spans.
Condemned to a miserable subterranean existence, the Kastrians, in Rokon's words, chose "final oblivion". They also elected to have the race bank destroyed as they knew Eldrad could have survived, due to the premature detonation, and might return with a desire to create an army and wage war. (TV: The Hand of Fear) The executioner, Mulkris, was assigned by Rokon to collect the fragments of Eldrad across space and place them into a stasis field. (AUDIO: Eldrad Must Die!) A number of booby traps were set around the thermal chambers in the hope Eldrad would trigger them. Rokon left recordings for Eldrad, explaining what had happened and mocking him as the "king of nothing". Eldrad saw these one hundred and fifty million years later when he returned to Kastria. Finding his species beyond restoration, he asked the Doctor to take him to Earth where he could rule the humans. With Sarah Jane Smith's assistance, the Doctor used his scarf to trip Eldrad, sending him plummeting into an abyss. (TV: The Hand of Fear) He managed to climb out, but in his solitude, went mad and began talking to the dust — the remains of dead Kastrians.
Mulkris destroyed, by her own estimations, "a hundred false Eldrads" on her journey. The eye of Eldrad was fully restored on Kastria by using the power of the Doctor's TARDIS. He was killed by the Eldrad restored from the hand who was in turn killed by Mulkris. In both instances the acid developed by Eldrad was used. Mulkris invited Vislor Turlough to join her in pursuit of further fragments, but he declined. (AUDIO: Eldrad Must Die!)
In 2008 Mr Smith mentioned that the then extinct Kastrians of Kastria were noted in his data base. He added silicon-based lifeforms like the Kastrian couldn't survive if they were turned into stone by a Gorgon. (WC: The Gorgon)