Rain of Terror

The Rain of Terror was a swarm of self-replicating, robotic creatures created by the Ulla people on Xirrinda. The creatures were originally created by the Ulla to rid them of their sins, but the technology quickly got out of control and practically destroyed the Ulla's advanced society.

They then travelled the galaxy, devouring whole planets and then multiplying thousands of times each second. Because of the size of their swarms they caused massive gravitational interferences. Ships couldn't lift from the ground when a swarm was nearby, and all machinery was turned off.

The Eleventh Doctor called them a machine, even though they resembled a species. After returning to Xirrinda many years after their creation, intent on devouring the planet, they were defeated by a machine the Ulla people had devised to destroy them in the first place. (PROSE: Rain of Terror)