The Cronquist were an aggressive psychokinetic species.
When a meteor, actually a segment of the Key to Time, passed through the Cronquist System, the Cronquist hitched a ride. When it got near Earth, they directed it to collide with a NASA shuttle. They controlled the only survivor, Millicent Ferril, to build a machine that would bring them to Earth. They were stopped by the Fourth Doctor and Romana when they dispersed the meteor. (AUDIO: Ferril's Folly)