Shalka

"We inhabit 80% of the worlds of the universe: those you regard as "dead""

- The Shalka

The Shalka are a race of sapient, xenophobic creatures with a collective hive mind, and are the rulers of a vast galactic empire encompassing over a billion worlds. However, their isolated nature means their empire is highly cryptic and generally unknown to the rest of the universe. The planets inhabited by the Shalka are typically regarded as dead wastelands by other species, who are unaware of them.

Biology
The Shalka are underground dwellers who need volcanic gases to live and can easily survive immersed in lava. They use sound waves to navigate (as their eyes are practically useless), as well as to stun and hypnotize their prey. They are described as "bioplasmic" creatures; therefore slaying or blasting their bodies doesn't kill them, and they are also capable of fusing their bodies together to form a new gestalt organism. Their bodies are generally worm-like, but can vary in size and form according to the individual's function within the hive: from the massive semi-humanoid Shalka Prime which leads them, to minuscule parasitic Shalka which can be implanted in humanoid hosts.

Culture and Society
The Shalka invade new planets hiding inside meteorites and take over the minds of the native population using their hypnotic screams. They later command their victims to scream at a frequency that causes the world's atmosphere to destabilize, rendering the surface uninhabitable and ensuring that no one else will be interested in the planet and the Shalka will be left alone with their otherwise dead world for themselves. The new planet is then connected via wormhole to the rest of the Shalka Empire.

It isn't clear how the Shalka continue to inhabit the planets after destroying their native biosphere. Presumably, they either bring in their own food sources or have no need to feed at all, taking all their energy from lithotrophy and/or geothermal sources.

Appearances

 * Doctor Who - Scream of the Shalka (2003)