The Consolidator was a huge spaceship constructed by the Time Lords, who imprisoned the Sild and the Quagulan on it, along with a large amount of dangerous technology that had been sealed up by other races across history. When they were finished with that stage of the project, the Time Lords were reluctant to destroy anything inside in case it would prove useful later. Having been sent into a black hole on the recommendation of the Time Lord who would become known as the Master, the ship was believed destroyed when it struck the edge of the hole by accident, but it was actually sent billions of years into the future, lost in space until it finally drifted into orbit around Praxilion. The inhabitants raided the Consolidator 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, then brought them to the Consolidator and used them to create time ruptures to obtain the Master who sent the original signal.
The Third Doctor freed the Master from his prison on Earth 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)