The Kastrians were an ancient race of silicon-based humanoids from the planet Kastria.
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.
Though the Kastrians had a wide variety of advanced technology, most of this was invented by Eldrad. This included the barriers on Kastria to protect it from powerful solar winds and the base on Kastria. Even the Kastrians' silicon-based forms were devised by Eldrad. Most of these advancements were either destroyed or degraded after Eldrad was killed. Kastrians also had a special crystal, generally in the form of a ring, which stored everything about them. This crystal stored the DNA and memories of the Kastrian, which allowed any remains to regenerate perfectly with enough radiation. The ring could also control the minds of other creatures, making them help the regeneration process. The ring could also pick up the physical traits of people who touched it, changing the Kastrians' form.
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. (PDA: 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 spaceship 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.
A hundred fifty 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 used the Doctor's TARDIS to take over Earth, but the Doctor destroyed him.