Children of Pyxis

Children of Pyxis are diminutive assassins deployed by Skorpius to kill the Doctor. They operate on negative time; so as soon as they leave one place, the time stream is fooled to believe that they've never been there. Thus no one will remember them, even reckoning their existence a myth. Though as a Time Lord sensitive to time, Romana and the Doctor are able to remember them. ( AUDIO: The Doomsday Contract)