The moon was shining bright upon the clearing, and there in the centre lay the unhappy maid where she had fallen, dead of fear and of fatigue. But it was not the sight of her body, nor yet was it that of the body of Hugo Baskerville lying near her, which raised the hair upon the heads of these three daredevil roysterers, but it was that, standing over Hugo and plucking at his throat, there stood a foul thing, a great, black beast, shaped like a hound, yet larger than any hound that ever mortal eye has rested upon.
The appearance of the old-time manuscript signals a shift in the narrative's format from Watson's straight-up reporting to the land of make-believe, manuscripts and a redoubled dubiousness. Mortimer reads the manuscript in a folk tale style, suggesting the importance of supernatural and fantasy in adding suspense to the plot.
Throughout the novel, Doyle is careful to distance his legendary detective from any implication of inaccuracy, even going so far as to couch the gathering of clues in a Watson-only space up in Devonshire. Here, the mystery that serves so important a function in the novel appears third-hand, and in a manuscript read by Mortimer. The reading of the manuscript serves both to allow the detective some critical distance and to make the mystery that much more ominous.