The Shalka were made almost entirely out of lava, possibly making them some kind of magma form, so they had no set form. They did appear once as large, worm-like creatures with cobra-like frills, but there were also Shalka of different sizes and shapes.
They lived underground, near where lava met metamorphic rock, breathing in the volcanic air, though Shalka Prime, their leader, was able to speak English and breathe oxygen. They communicated by screaming and used sound as a weapon and a means of controlling "lower alien creatures", like humans, and to accomplish other tasks.
Sometimes, the Shalka merged themselves together to form a larger, lumbering and (seemingly) less intellegent body called the Shalka Conglomerate.
Their technicians had extra arms and brains studded down their backs.
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. (NOTVALID: Scream of the Shalka)