Victoria

“Creatures from another world, only statues when you see them. Lonely Assassins, that's what they used to be called. No one quite knows where they came from, but they're as old as the Universe, or very nearly. And they have survived this long because they have the most perfect defence system ever evolved. They are Quantum Locked. They don't exist when they are being observed. The moment they are seen by any other living creature they freeze into rock. No choice, it's a fact of their biology. In the sight of any living thing, they literally turn into stone. And you can't kill a stone. Of course, a stone can't kill you either, but then you turn your head away. Then you blink. Then, oh yes, it can. That's why they cover their eyes. They're not weeping, they can't risk looking at each other. Their greatest asset is their greatest curse. They can never be seen. Loneliest creatures in the Universe. And I'm sorry. I am very, very sorry. It's up to you now. Don't blink. Don't even blink. Blink and you're dead. They are fast. Faster than you can believe. Don't turn your back, don't look away, and DON'T blink. Good luck.”
― Steven Moffat