Sild

The Sild were a race that could control other beings by attaching to their bodies and linking into their nervous systems using travel machines resembling silver crabs. They would arrive on a planet, take over the native population and move on when they had exhausted the resources.

Biology
A race of miniscule beings resembling seahorses, the Sild can be easily killed by smashing them under foot. They are incapable of land movement without travel machines. The main Sild threat comes from their ability to directly link with other lifeform's nervous systems. This process is almost always fatal to the host - causing nerve damage even in Time Lords.

History
The Time Lords considered them such a threat that they imprisoned the entire race on the spaceship Consolidator, along with a number of other dangerous devices.

The Consolidator travelled for billions of years until it finally drifted into orbit around Praxilion. The inhabitants raided the gigantic ship for its technology. After six thousand years, they reached the area where the Sild were confined and unintentionally released them.

The Sild fought the Praxilions for half a million years. Then they picked up a distress signal that was sending to a future self to free him from incarceration by UNIT. They captured hundreds of incarnations of the Master, rendered them unconscious, and used them to create time ruptures to obtain the Master who sent the original signal.

The Third Doctor freed the Master before the Sild could capture him. They travelled to Praxilion in the far future and destroyed the Consolidator, and all the Sild onboard. (PROSE: Harvest of Time)