This is a faceless fear, first introduced in the episode "Silence in the Library." Unlike the many other Doctor Who enemies, there was no way to fight it or to protect yourself from it. The only sure way out was to run. During the course of the episode, the Doctor tried many different tactics to fight the Vashta Nerada, but all ended in failure.
These creatures travel in dark swarms through the air. Their appearance resembles shadows, and they have learned to use that feature as a disguise in order to sneak up on unsuspecting prey. They work like piranhas, attacking in swarms and stripping their victims to the bone in fractions of a second. Unlike piranhas, these creatures are very intelligent. They are even able to inhabit and operate a space suit in order to make it walk in a human-like fashion, as well as communicate. These shadowy beings typically live and hunt in forests. According to the Doctor, they are the reason humans and other creatures have a seemingly-irrational fear of the dark, and yes, they also live on Earth.
