The Weeping Angels

Although you probably won't encounter any of these ancient aliens, I wanted to write about them anyway, and you'll be prepared if for some strange reason you do.
Weeping Angels. The Lonely Assassins.

But they're not weeping. They're quantum locked, which means that when you observe them, they simply cease to exist. They cover their eyes because if they see each other, they're frozen in place as statues for all eternity, unable to blink, unable to look away. Their greatest asset is their greatest weakness. If they touch you, you go back in time to live yourself to death, while they feast off your lost days. But if you look at them, they're stone. To use the words of the Doctor, "And stone can't kill you. But then you look away, and oh yes it can."

So don't blink.

The Doctor: Listen—your life could depend on this—don't blink. Don't even blink. Blink and you're dead. They are fast, faster than you could believe. Don't turn your back, don't look away, and don't blink. Good luck.

(Seen in Doctor Who episodes Blink, The Time of Angels, and Flesh and Stone)