Vashta Nerada

Vashta Nerada were swarming, carnivorous beings. The name meant "the shadows that melt the flesh".

Biology
The Vashta Nerada were microscopic beings that lived in swarms, thousands strong. Individually, Vashta Nerada were not a threat. They were very small; the Tenth Doctor claimed that the dust specks visible in bright sunlight were single Vashta Nerada or small swarms, not large enough to be a threat to most life. In large numbers, they could strip a creature to its bare bones in milliseconds. Vashta Nerada swarms were sentient and two spacesuit-inhabiting swarms developed the ability to communicate with the Doctor through the spacesuits' Data Chips. They also coordinated their efforts and made the spacesuits walk, though only with some difficulty; they walked slowly in a 'zombie-like' fashion.

The Vashta Nerada lived in the darkness. They cast shadows when they entered the light - even to the extent of stopping the reach of light from fission lamps. On their own, outside of the darkness, a Vashta Nerada swarm looked like a shadow cast by nothing. They mimicked the shadows of their prey to get close, which meant staying in the light was the only option to escape. If someone had an extra shadow, it was already too late. Sonic technology could detect the difference between a Vashta Nerada swarm and an ordinary shadow. (DW: Silence in the Library/Forest of the Dead) The Doctor believed they had no weaknesses (other than the light) and the only thing to do was to run. (DW: Silence in the Library, WEB: Captain Jack's Monster Files)

Life cycle
The Vashta Nerada lived on almost every planet which had meat, including Earth. They were born as microscopic spores in trees and lived their lives in forests. These spores were resilient enough to hatch even if the tree was converted into paper and books. It was said they could hatch and form into a swarm in minutes. (DW: Forest of the Dead)

History
Vashta Nerada existed since the early Universe, and spread to many planets. While their swarms were not common, they inspired a fear of the dark in most species, since any shadow could be made of Vashta Nerada. According to the Doctor, this was why young children were afraid of the dark. They knew, at the back of their minds, that the Vashta Nerada were there. (DW: Silence in the Library/Forest of the Dead) Normally, the Vashta Nerada fed off roadkill. (VG: Shadows of the Vashta Nerada')


 * How the spores originally got to different planets is unknown.

On Earth, the Vashta Nerada were largely harmless and benign. (VG: Shadows of the Vashta Nerada) They survived by scavenging road kill, but the Doctor speculated that they were behind the disappearance of hikers. (DW: Silence in the Library/Forest of the Dead)

In the 23rd century, a temporal rift resulted in hostile Vashta Nerada arriving in the underwater city Poseidon 8 on Earth, along with a giant shark-like being. The Eleventh Doctor and Amy defeated some of the Vashta Nerada by luring the diving suits they were using into a brightly-illuminated area; The Doctor later closed the rift and returned the Vashta Nerada to their original Planet. (VG: Shadows of the Vashta Nerada)

In the 50th century one planet suffered a large infestation. The Vashta Nerada usually live in their forests, but when some of the trees were cut down and made into books, to be put in the the Library, some of the spores got trapped henceforth. The Library was attacked by over a "million million" Vashta Nerada; spores inside its books hatched. The four thousand and twenty-two people on the planet could not be evacuated and were 'saved' in its computer's hard drive. The computer created a virtual world for them to live in and the planet was empty for a hundred years until an archaeological expedition, the Tenth Doctor and Donna Noble landed on the planet.

Though much of the archaeological team was consumed by the Vashta Nerada, the Doctor convinced the Vashta Nerada to stop and give him a chance to return the people stored in the Library's computer. They agreed to give him a day after reading about him in the Library. The humans were rescued and left the planet to the Vashta Nerada. (DW: Silence in the Library/Forest of the Dead.) 

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