Vashta Nerada
The Vashta Nerada were first seen in Silence in the Library (Series 4 : Episode 8) and Forest of the Dead (Series 4 : Episode 9). The Vashta Nerada are microscopic beings that live in shadows, creating the fear of the dark for many species. Alone, the Vashta Nerada aren’t harmful to living creatures, but when a swarm get together they’re able to strip the meat from the bone in seconds, for which the Doctor described them as “piranhas of the air”. Two spacesuits inhabited with Vashta Nerada were able to walk, in a zombie fashion and with difficulties, as well as being able to communicate with the Doctor through the use of data chips attached to the spacesuit. The people who had worn the spacesuit were reduced to bones.
The Vashta Nerada live in the darkness of trees, where they would breed, and can be found on any planet with living organisms that have meat to eat. It is said that they survive on Earth by eating road kill. They could also hatch even if the tree had been processed into another item, such as paper for books. When they entered the light, they cast a shadow, causing many species to have a fear of the dark as the only safe thing to do is stay in the light. The Vashta Nerada used this trick to get close to their victims, posing as shadows and moving closer to their victim until they could consume it, however sonic technology is able to differentiate between a Vashta swarm and an ordinary shadow. The Doctor advised that the best way to beat the Vashta Nerada was to run, as he believed them to have no weaknesses.
The Vashta Nerada arrived on the planet The Library through the wood used to build it, and the paper books inside it. When they hatched, there was over a million million Vashta Nerada present. They swarmed The Library causing the computer system C.A.L to unsuccessfully evacuate the 4022 people on the planet at the time the Vashta Nerada attacked. The Doctor forced them to let him get the people off the planet safely after making them read about him in the books on The Library. They agreed to give him a day to save them all, so whatever was written about the Doctor evidently scared them enough to let him do what he wanted. After the Doctor evacuated the people and left himself, the Vashta Nerada were free to continue roaming The Library.