Vashta Nerada, your second shadow
With a name like Vashta Nerada it sounds like some kind of uncomfortable chaffing in the nether regions. The real Vashta Nerada is a little more permanent but just as discomforting.
Vashta Nerada are swarms of nearly microscopic parasites that are usually harmless, according to the Doctor.
They exist primarily in the dark, which is why almost every species instinctively fears the dark. Mostly harmless means sometimes deadly.
In Silence in the Library/Forest of the Dead they come across one of the rare times as the Vashta Nerada have taken out a whole city, potentially a whole planet.
These are also the episodes where the Doctor meets River Song for the first time. Spoilers: she dies. But as we find out later, her timeline is in reverse to the Doctor’s and it’s not the last time we see her.
The Tenth Doctor, and Donna, find a group of explorers in a 51st century library. Then people start dying.
It’s not an immediate and obvious attack like in Aliens. The group doesn’t even realize when someone has been killed until it’s past too late. Some of the people in the group are wearing space suits, and the Vashta Nerada have the ability to keep the space suit filled out and give the appearance of someone just standing around. The tell that the Doctor figured out is when someone had a second shadow.
Despite being nearly microscopic, the Vashta Nerada cast a shadow wherever they go. Before attacking it would look like a shadow cast by nothing. Once you have a second shadow, it’s already too late for you.
That seems unfair. You can see a Dalek or a Cyberman coming, you may not stand much of a chance to get away, but you can still run. The only option Vashta Nerada gives you is to stay in the light. Even then, you have to be looking for shadows.
It seems like if they want you, they’re going to get you and you’re not even going to see it coming. Maybe that’s not such a bad thing…