Vashta Nerada

Vashta Nerada, literally, "the shadows that melt the flesh", were swarming, carnivorous beings which were found on most planets.

Biology
The Vashta Nerada were microscopic beings that lived in swarms, thousands strong. Individually or in small numbers, Vashta Nerada were not a threat. They were very small; the Ninth Doctor claimed that some of 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. (DW: The Reckoning of Shadows)

The Vashta Nerada lived in the darkness. They cast shadows when they entered the light — even 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 way to escape. Light also slowed them down somewhat, but if the swarms were great enough, the effects of light would be minimised and overcome quickly. If someone had an extra shadow, it was already too late. The Doctor believed they had no weaknesses, and the only thing to do was to run. (DW: The Reckoning of Shadows)

Besides the swarm in the Library, the Doctor indicated that the Vashta Nerada were generally harmless. Existing on over a billion planets including Earth, the Vashta Nerada inspired a fear of the darkness but were nowhere near as aggressive as they were in the Library. The Doctor stated that he'd never seen a swarm as large or as aggressive as the Library swarm. However, this unusual aggressiveness came from the fact that the books were printed with paper from the trees of the Vashta Nerada's forests and thus they saw the Library as their "forest". (DW: The Reckoning of Shadows)

History
To be added. (DW: The Reckoning of Shadows