The Vashta Nerada are ravenous hunters able to strip the meat from a body with a frightening speed and efficiency. They tend to avoid light, often concealed in patches of dark that resemble shadows, hence the general warning to 'Stay out of the shadows' if you suspect these creatures are close.
Even species that are not consciously aware of the threat posed by the Vashta Nerada seem to sense their menace. As the Doctor once observed, 'Almost every species in the universe has an irrational fear of the dark. But they're wrong. Because it's not irrational. It's Vashta Nerada.'
Stay out of the shadows!The Doctor
In their natural state the Vashta Nerada are tiny but when they gather in large numbers they're able to animate objects, including the remains of humans. Their adaptability is possibly one reason for their universality; they have encountered the Doctor in different centuries and their race has blighted numerous planets across the cosmos.