Kastrian

The Kastrians were an ancient race of silicon-based humanoids from the planet Kastria.

Biology
Kastrians were large humanoids with a rocky exterior and a large crystalline structure on their heads. They had some mental powers, such as telepathy and a type of mental attack (which required their rings to function). Kastrians had the ability to absorb radiation in order to regenerate their body, which allowed them to regrow their entire body if they have enough energy. (DW: The Hand of Fear)

Technology
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 also the base on Kastria. Even the Kastrians' silicon-based forms were themselves devised by Eldrad. Most of these advancements were either destroyed or degraded after Eldrad was killed.

Kastrians also had a special crystal (which was 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 be regenerated perfectly with enough radiation. The ring could also control the minds of other creatures, making them help the regeneration process. Despite this, the ring could also pick up the physical traits of the people who touched it, causing changes in the Kastrian's form. (DW: The Hand of Fear)

History
The Kastrians were an ancient race from around 150 million B.C.. During the Millennium War, they attempted an assault 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)

Presumably, at this point Eldrad began making improvements in Kastrian society, such as giving them their silicon-based bodies and setting up the barrier to protect their world. Unfortunately, Eldrad was a cruel ruler, and he attempted to create an army to conquer the universe. The Kastrians revolted and tried to stop his actions. He then destroyed the barriers protecting the planet, though the Kastrians led by King Rokon were able to capture him. the spaceship he was in was destroyed, the hand of Eldrad was blasted to Earth along with his ring. The remaining Kastrians were a doomed species, since the only option left was a miserable underground existence. The Kastrians elected to give up and destroy all of the memory-crystals in the race banks, stopping the regeneration of any Kastrians.

150 million years later, Eldrad's hand was accidentally uncovered from 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 was able to regenerate his form, though his ring imprinted off Sarah and was modelled after her. He was able to convince the Doctor to return in to Kastria, but he refused to take him into the past. At this point, he learned that the other Kastrians had died out. Though he planned to used the Doctor's TARDIS to take over Earth, the Doctor was able to destroy him. (DW: The Hand of Fear)