Kroton

The Krotons are a sapient species of tellurium-based lifeforms.

Biology and technology
Crystalline creatures, the Krotons also appear to be part robotic and may in fact be a type of cyborg. Masters of advanced technology, the Krotons travel in spacecraft called dynatropes, which are powered by mental energy. Biologically immortal, they can melt down into a slurry liquid state of stasis. They can be killed by an impure form of sulfuric acid, released into their tellurium-based life support system. Krotons prefer dark environments and are unable to see properly in abundant light.

History
At some point, a Kroton dynatrope crashed on the homeworld of a primitive humanoid species known as the Gonds. For many generations the two surviving Krotons - preserved in their liquid states inside their heavily equipped ship - controlled and restricted the scientific development of the Gonds and were viewed almost as gods by them. Occasionally the Gond considered the most brilliant student was selected by the Krotons to become their "companion" and enter the ship to never come back. As it turns out, the Krotons were actually draining their mental energies to power the dynatrope and subsequently destroying them. When the Doctor arrived on the Gond planet, he witnessed this and revealed the truth to the Gonds, who organized themselves to rebel against their rulers. The two Krotons returned to their solid form but were ultimately killed, leaving the Gonds free.

Appearances

 * Doctor Who season 6 (second Doctor); "The Krotons"