Count your shadows

Count your shadows, or they'll eat you.

_{The Vashta Nerada are an alien race of microscopic particles. When in extremely thick concentration, they make up darkness and can strip away muscles, organs, tissue, and skin from a living being in less than a second. They are found in every known corner of the universe, including the Earth (the dust particles in sunbeams).
In the Library, they latched onto a victim (giving them two shadows) before taking over and eating them.}
Episodes 8 and 9, Silence In The Library and Forest Of The Dead. New!Who, Season 4.

"Every species has an irrational fear of the dark. Only maybe it's not so irrational. It's Vashta Nerada." -The Doctor

Vashta Nerada and other aliens brought to you by Doctor Who. Watch it.