Vortex Amoeba were amoeba that lived in the Time Vortex.
The Ninth Doctor stated that he hadn't seen them for centuries, before disturbing them, sending one into Halifax's past, and himself into the 21st century. It absorbed the energy from the people executed using the Halifax gibbet and influenced Annalise Avenly's mind, as well as previous generations of her family. (AUDIO: The Running Men)