The Doctor: The 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 defense 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.
The Empty Child: Are you my Mummy?
The Silent: The Silents, Doctor. We are the Silents. And Silence will fall!
Head Librarian’s Message: Run. For god’s sake run. Nowhere is safe. The library has sealed itself. We can’t— Oh, they’re here. Count the shadows. For god’s sake, remember. If you want to live, count the shadows. Message ends.