Kastrian The Kastrians were natives of the planet Kastria . They were silicon - based lifeform s , 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 wind s , led the Kastrians to choose suicide rather than be consigned to life underground on their desolated world . Biology . Eldrad developed a crystalline silicon form for the physical needs of the Kastrian race . It was humanoid , with a rock - like texture . Kastrians could restore themselves through irradiation . This allowed them to return to life even if select particles survived . The Fourth Doctor noted that this made them " very difficult to kill " . 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 ! " ) Technology . Eldrad produced several technological marvels for the Kastrian people . He built the spatial barrier s to protect the planet from solar wind s , 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 span s 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 ring s could also be used to restore Kastrians to life , with their individual genetic codes being retained within . ( AUDIO : " Eldrad Must Die ! " ) History . 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 computer s were turned against them . ( PROSE : " The Quantum Archangel " ) To shelter from the solar winds , Kastrians resided in the thermal chamber s , 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 trap s 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 human s . 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 ! " ) Other information . 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 " )