Kastrian

The Kastrians are a race of silicon-based, humanoids from Kastria, first shown in The Hand of Fear.

Biology
Kastrians appear to be large humanoids made out of black rock. They have a large crystalline structure on their heads. They have some mental powers, such as telepathy and some type of mental attack. Kastrians have the ability to absorb radioactive energy in order to regenerate their body, which makes them practically immortal.

Kastrians also have a special crystal (which Eldrad had as a ring) which store everything about them. This crystal stores the DNA and memories of the Kastrian, which allows any remains to be regenerated perfectly. This crystal can also recreate the Kastrian in a form based off other lifeforms.

History
The Kastrians are an ancient race from around 150 million years ago. They were originally not Silicon-based, but Eldrad was able to create the new forms. Eldrad also created barriers to try to make Kastria more hospitable, which protected the planet from the solar winds.

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, but was eventually annihilated for his actions. One hand of Eldrad was blasted to Earth along with the ring. The Kastrians were a doomed species anyway, since the only option was a miserable underground existence. The Kastrians elected to give up and destroy all of the memory-crystals, stopping the regeneration of any Kastrians.

As of the 20th century, the Kastrians were extinct, with Eldrad being the last survivor.