The Shalka Confederacy spread across a billion worlds, invading planets using meteorites and then linking them to the rest of the empire by wormholes which they converted from black holes. The Shalka used their screams to take over the planets' beings and make them scream to disrupt the atmosphere and scorch the planet's surface. With the surface completely charred, it made other species think that the planet's population had done it themselves, while the Shalka lived comfortably under the earth. In essence, the Shalka were the death factor of the universe.
The Ninth 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 frequency. (WC: Scream of the Shalka)
