Children of Pyxis
From Tardis Wiki, the free Doctor Who reference
Children of Pyxis were diminutive assassins deployed by Skorpius to kill the Doctor. They operated on negative time; so as soon as they left one place, the time stream was fooled to believe that they had never been there. Thus, no one remembered them, even reckoning their existence a myth. Though a Time Lord and therefore sensitive to time, Romana and the Doctor were able to remember them. (AUDIO: The Doomsday Contract)
