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