The Xylok were a mysterious crystalline race.
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. Due to their crystalline and computer-like nature, they were also vulnerable to 21st century Earth computer viruses.
Roughly 60 million B.C., a Xylok ship or meteorite crashed into the planet Earth. All of the Xyloks were trapped in Earth's crust and were unable to escape. In 1883, during the volcanic eruption at 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. The Xylok communicated with her via Sarah's laptop and convinced Sarah Jane that he could help her defend the Earth. It designed an advanced computer system for her to use as a body which she kept hidden in the attic of her home. 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.
Later, Mr Smith returns to his original function. He is integral, in combined efforts with Torchwood Three and Martha Jones, in contacting the Doctor when Daleks invade Earth.
Behind the ScenesEdit
The Xylok have some similarity with another crystalline race, the Krotons.