Vashta Nerada
The piranhas of the air. The Vashta Nerada. Literally, the shadows that melt the flesh.
Vashta Nerada were swarming, carnivorous beings. The name meant "the Shadows that melt the flesh". (DW: Silence in the Library / Forest of the Dead)
Biology
That's not darkness down those tunnels. This is not a shadow. It's a swarm. A man-eating swarm.
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 species across the Universe. Vashta Nerada swarms hunted in forests because they hatched from their spores there.
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.
Despite being microscopic as individuals, Vashta Nerada swarms were sentient and two spacesuit-inhabiting swarms developed the ability to communicate to the Doctor through the spacesuits' Data Chips. (DW: Silence in the Library / Forest of the Dead) On that note, they appear to have a collective conciousness, as the Doctor can communicate with one, and it can relay the message to the rest of the planet to research him. Not to mention the fact the one possessing Anita's suit could use the communicator right off the bat.
- Elemental Shades from the Howling Halls appeared similar to Vashta Nerada, but worked in different ways (DW: Love & Monsters). It is unknown if the two were related in any way.
Life Cycle
Vashta Nerada were born as microscopic spores in trees. These spores could still hatch even if the tree was processed, including if the wood was converted into paper and into books. (DW: Forest of the Dead)
History
These are our forests. We are not kind.
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 often those that had been killed on the many roads.
In the 50th century, however, one planet suffered a large infestation. The Library was attacked by quadrillions (it would be difficult to estimate just how many there were as the computer capped at 1 trillion) of Vashta Nerada due to the spores inside the books' paper hatching. 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 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) When asked how to deal with them he stated the weak points of the Daleks and Sontarians(being eystalks for the daleks and the back of the neck for the sonatarans) but advised the team to "run, just run" from the Vashta Nerada. Curiously later on the Doctor was able to make a sapient swarm of Vashta Nerada retreat merely by telling them to look him up in the Library's records, suggesting they may indeed have a weakness that he could discover or that they were extremely intimidated by the information of what he had done.
