The Quiet Ones had developed technology that captured starlight to power their city. When the planet was caught in the gravity of a pulsar, they were threatened with extinction. They used their technology to turn themselves into pure energy. They appeared invisible apart from their shadows because they moved at a speed humans could not see.
In 2724, they unwittingly killed Fifth Galactic Surveyor Corps members Woodbridge and Gough because the humans from Earth existed in a different time zone to them and were fragile to the temporal shock. The Second Doctor spent years with the Quiet Ones agreeing to a truce in which the surveyor team could continue to learn about the Quiet Ones, if they did it more quietly. The Doctor also modified the instruments and systems so they made less noise, and he allowed the team to be able type messages to the Quiet Ones. (AUDIO: Shadow of Death)