6. Silence In The Library/Forest Of The Dead
This is another monster we never actually got to see, the unspellable Vashtanarada. Not every shadow, but any shadow. "Not everyone comes back out of the dark." They're on every planet, including this one. They can strip bones in seconds. Add to that the macabre nature of their communication--the souls and skeletons of dead people--and you've got a cocktail of horror, as the Doctor, River, and assorted company are chased around a library full of dead faces. Every basic question was answered by ghosts. In that library, even the computers were full of dead people. In the end, the Vashtanarada were just animals, just creatures swarming in their forests. That didn't make them less scary, or the damage they caused less real. But even they feared the Doctor, and that's why they were defeated. They were scary, and they were defeated, and it's over. Unless...have you got two shadows?