Kastria was a desolate planet inhabited by the Kastrians.
When the Kastrian fleet was utterly destroyed, Kastria was ravaged as the Mad Mind of Bophemeral turned their computers against them in the Millennium War. (The Quantum Archangel)
Kastria was barren, cold and constantly ravaged by solar winds. A Kastrian named Eldrad created a barrier to protect Kastria and new silicon-based bodies to deal with the environment. These efforts were for naught, since the barrier was destroyed and the Kastrians rendered extinct.
One hundred fifty million years later, the Fourth Doctor and Sarah Jane Smith returned Eldrad to Kastria, now a dead planet. (The Hand of Fear)
Kastrians were large humanoids, with a rocky exterior and large crystalline structures on their heads. They had some mental powers, such as telepathy and a type of mental attack which required their rings to function. Kastrians could absorb radiation to regenerate their bodies. They could regrow their entire body if they had enough energy. (The Hand of Fear)
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. (The Quantum Archangel)
It was presumably at this point that Eldrad began making improvements in Kastrian society, giving them their silicon-based bodies and setting up the barrier to protect their world. Eldrad was a cruel ruler, and he attempted to create an army to conquer the universe. When the Kastrians revolted, he destroyed the barrier protecting the planet. The Kastrians, led by King Rokon, captured him and executed him by destroying his spacecraft with him on it. (However, his hand, along with his ring, survived and ended up on (Earth). The remaining Kastrians were doomed, their only option a miserable, underground existence. They gave up and destroyed all of the memory-crystals in the race banks, stopping their regenerations — an act which would also be a precaution against Eldrad’s possible return. (The Hand of Fear)
In 1903, after receiving a wealth of information from the future, Grigori Rasputin foresaw creatures made of silicon. (The Wanderer)
In the late 20th century, 150 million years later, Eldrad’s hand was uncovered in a quarry and picked up by Sarah Jane Smith. His ring overpowered her mind, forcing her to seek out the Nunton Experimental Complex for a source of radiation. He regenerated, though his ring imprinted on Sarah, and his new form was modelled on her. He convinced the (Fourth Doctor to return her to Kastria, although he refused to take Eldrad to the past. On Kastria, he learned his race had died out. He planned to use The Doctor’s TARDIS to take over (Earth, but The Doctor destroyed him. (The Hand of Fear)