The Darkness (The Well-Mannered War)

 ''Not to be confused with "The Darkness", the name given to the after effects of the detonation of the reality bomb. You may also be looking for the Beast, also known as the Darkness.'' 

The Darkness was a hive mind of insects.

Though physically the darkness looks no different from a swarm of regular flies, they evolved a type of hive mind called the Onememory. They had some psychic powers, but they needed a large enough swarm in one place for it to be effective.

At some point in Earth's future, flies evolved the Onememory and started their feeding cycle. Using their mind-controlling powers, they would cause wars and chaos and then feed on the carrion produced.

They particularly liked to feed on humans, a self-destructive species. They left Earth before its destruction in a pyramidal craft, and travelled the universe, alternating between times of feeding and times of sleep.

They claimed to have feasted on over 60 billion species and adapted parts of their anatomy for their biological-based craft. The Darkness discovered the planet Metralubit and its human population, which they choose as the next feeding ground.

Most of the Darkness was wiped out by the Time Lords, with the last swarm being destroyed in the 58th Segment of Time by the Fourth Doctor. (PROSE: The Well-Mannered War)