Monster Files: Vashta Nerada

The Vashta Nerada were microscopic beings that lived in swarms, thousands strong. These swarms were able to strip a creature to its bare bones in milliseconds and the Doctor once described them as "piranhas of the air". While these swarms were not common, they inspired a fear of the dark in most species across the Universe, since any shadow could be made of Vashta Nerada. Both the Doctor and Jack Harkness believed they had no weaknesses and that the only thing to do was to run.

The Vashta Nerada lived in the darkness and cast shadows when they entered the light - even to the extent of stopping the reach of light from fission lamps. They would mimic the shadows of their prey in order to get close, which meant staying in the light was the victim's only option for escape.