Shalka
The Shalka are an alien race made almost entirely out of lava, possibly making them some kind of magma form, and so they have no set form, though they did appear once as large worm-like creatures with cobra-like frills, but there were also Shalka of different sizes and shapes.
They live underground, near where lava meets metamorphic rock, breathing in the volcanic air, though Shalka Prime, their possible leader, was able to speak english and breathe oxygen. They communicate by screaming and use sound as a weapon and a means of controlling "lower alien creatures", like humans, and to accomplish other tasks.
The Shalka Confederacy spreads across a billion worlds, invading planets using meteorites and then linking them to the rest of the empire by wormholes which they convert from black holes. The Shalka use their screams to take over the planet's beings and make them scream to disrupt the atmosphere and scorch the planet's surface. So while the surface is completely charred, causing other species to think the planet's population did it themselves, the Shalka live comfortably under the earth. In essence, the Shalka are the death factor of the universe.
The Doctor encountered the Shalka on Earth in 2003 when they attempted to destroy the surface, but the Doctor was able to blow them up by singing on their freguency.(WC: Scream of the Shalka).
Their technitions have extra arms, and brains studded down their backs, and
Behind the Scenes
As the version of the Doctor who fought the Shalka does not exist in established canon, the Shalka themselves may not either.
