Doctor: Almost every species in the Universe has an irrational fear of the dark. But they're wrong. 'Cause it's not irrational. It's Vashta Nerada..// Donna: What's Vashta Nerada?..// Doctor: It's what's in the dark. It's what's always in the dark.
SILENCE IN THE LIBRARY. Vashta Nerada are swarming, carnivorous beings. The name meant "the shadows that melt the flesh". These microscopic beings live in swarms, thousands strong. Individually, they aren't a threat. They're very small; the Tenth Doctor claimed that some of the dust specks visible in bright sunlight are 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.