Vashta Nerada
Vasta Nerada were swarming, carnivorous beings. The name meant "the Shadows that melt the flesh". (DW: Silence in the Library / Forest of the Dead)
Biology
The Vashta Nerada were microscopic beings that lived in swarms, thousands strong. Swarms of them lived on almost every planet which had organisms with meat. While these swarms were not common, they inspired a fear of the dark in most races across the Universe.
These swarms were able to strip a creature to its bare bones in milliseconds. The Doctor once described them as "piranhas of the air". The swarms 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.
Vashta Nerada were capable of latching their shadows onto a victim's shadow and changing to appearance of their shadow to that of their victim's. This would cause the victim to have two shadows. If the swarms were discovered, they could enter at least through a thick spacesuit and devour the body. They could then use the suit as a vessel to attack more prey. (DW: Silence in the Library / Forest of the Dead)
History
Vashta Nerada had existed since the early Universe, and spread to many planets across the Universe. They were rare, and on some planets such as Earth they survived by scavenging meat from dead animals.
In the 51st century, however, one planet suffered a large infestation. The Library was attacked by trillions of Vashta Nerada. There were no survivors, and the planet was empty for 100 years until an archaeological expedition and the Doctor with Donna Noble landed on the planet in the same day.
In this time, the Doctor was given advice on how to deal with the Vashta Nerada. This was; "Count the shadows." The Doctor later gives the Archaeological team a caution to "Stay out of the shadows." (DW: Silence in the Library / Forest of the Dead)
Notes
In the Series 2 episode Love and Monsters when the Doctor is telling Elton Pope about the day Elton first saw him, he says :"There was a shadow in your house. A living shadow" to explain why he was there. It is possible that this "living shadow" was infact a large swarm of Vashta Nerada, although most likely not as large or aggressive as those seen in Silence in the Library and Forest of the Dead as a small glimpse of Elton's dead mother is seen in the flashback, still with all her flesh attached. This might mean that the Vashta Nerada can kill a person without actually stripping them of their flesh. The shadow is not Vashta Nerada, but an Elemental Shade from the Howling Halls, as explain in the episode. The two ceatures may be similar.
