The Doctor, Donna and others in “Silence in the Library”

That's not darkness down those tunnels. This is not a shadow. It's a swarm, a man-eating swarm. [throws a chicken leg into the shadow, which is immediately reduced to mere bone] The piranhas of the air. The Vashta Nerada. Literally, "the shadows that melt the flesh". Most planets have them, but usually in small clusters. I've never seen an infestation on this scale, or this aggressive.

What do you mean, most planets? Not Earth?

[nodding] Mmm, Earth. And a billion other worlds. Where there's meat, there's Vashta Nerada. You can see them sometimes, if you look. The dust in sunbeams.

[unnerved, in denial] If... they were on Earth, we'd know.

Nah. Normally they live on roadkill. But sometimes people go missing. Not everyone comes back out of the dark.

Every shadow?

No. But any shadow.

So what do we do?

Daleks, aim for the eyestalk. Sontarans, back of the neck. Vashta Nerada? Run. Just run.