Vashta Nerada were swarming, carnivorous beings.
The Vashta Nerada were microscopic beings that lived in swarms, thousands strong. On their own, Vashta Nerada were not a threat. They were very small; the Doctor once claimed that the dust specks visible in bright sunlight were single Vashta Nerada or a small swarm not yet large enough to be a threat to most life. Once they were in sufficient numbers, they were able to strip a creature to its bare bones in milliseconds and the Doctor described them as "piranhas of the air".
The Vashta Nerada lived on almost every planet which had organisms with 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 processed, including if the wood was converted into paper and into books, which explains why so many were present on the planetary Library. It was said that they were able to hatch and form into a swarm in minutes.
Vashta Nerada swarms were sentient and two spacesuit-inhabiting swarms developed the ability to communicate to the Doctor through the 50th century's spacesuits' Data Chips. They were also able to coordinate their efforts and make the spacesuits walk, though only with some difficulty, and could walk slowly in a 'zombie-like' fashion.
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. On its own, outside of the darkness, a Vashta Nerada swarm would look like a shadow cast by nothing. They would mimic the shadows of their prey in order to get close, which meant staying in the light was the only option for escape. If someone had an extra shadow, it was already too late for them. Sonic technology is able to detect the difference between a Vashta Nerada swarm and an ordinary shadow.
The Doctor believed they had no weaknesses (other than the light) and that the best thing to do was to run.
Vashta Nerada had existed since the early Universe, and spread to many planets across the Universe. 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. They were rare, and on some planets, such as Earth, they survived by scavenging meat from road kill.
In the 23rd century, a Crack resulted in a group of hostile Vashta Nerada being transplanted from their home to arrive in the underwater city Poseidon 8 on Earth, along with Percy the Shark. Fortunately, the Sixth Doctor and Bob Price were able to defeat some of the Vashta Nerada by luring the diving suits they were using into a brightly-illuminated area, the Doctor later closing the crack and returning the Vashta Nerada to their original planet.
In the 50th century one planet suffered a large infestation. The Library was attacked by over a "million million" (1,000,000,000,000) Vashta Nerada, due to the spores inside the books' paper hatching. The 4,022 people were unsuccessfully evacuated by the Library's computer and the planet was empty for 100 years until an archaeological expedition lead by Dr. Daphne Downs and the Fifth Doctor landed on the planet.
Though much of the archaeological team was eaten by the Vashta Nerada, the Doctor was able to convince them to stop and give him a chance to return the people stored in the Library's computer. They agreed to give him a day, in which time the humans were rescued and left the planet to the Vashta Nerada.
Time and Space: Series 5Edit
- "Saved from the Books"
Time and Space: Series 6Edit
- "Shadows of the Deep"
- "Flatmates" (Mentioned only)