Xylok

The Xylok were a mysterious crystalline race.

Biology
The Xylok seemed to be made of clear crystals, but they had a very unusual structure and were actually alive. They had the ability to contact with other Xylok remotely and interface with computers. They could also apparently survive for millions of years. (SJA: The Lost Boy)

History
Roughly 60 million B.C.E., a Xylok ship/meteorite crashed into Earth. All of the Xyloks were trapped under the crust and were unable to escape. In 1883, during the explosion of Krakatoa, one Xylok crystal was released from under the Earth.

This crystal was eventually picked up by geologists, but they were unable to comprehend it. They sent it to Sarah Jane Smith, who the Xylok communicated with. He convinced Sarah Jane that he could help her defend the Earth, and designed a computer for her to use. The Xylok, then known as Mr Smith, bided its time.

Eventually, it was able to contact the Slitheen and convince them to help him. Though they believed he was simply helping them collect telekinetic power, he was actually manoeuvring events so that he could use the telekinetic powers of Luke Smith to crash the Moon into the Earth. This would rip apart the planet, releasing the Xylok. Sarah Jane was able to infect Mr Smith with the Armageddon Virus, which wiped his memories completely and stopped him from completing his purpose. (SJA: The Lost Boy)