What does stapleton reveal that shocks Watson in the hound of baskervilles?

Stapleton reveals to Watson that he is the mastermind behind the hound of Baskerville. He shocks Watson by demonstrating his knowledge of the moor, the legends surrounding it, and his ability to control the hound using a chemical solution. Stapleton's revelation highlights the complexity and duplicity of his character, as he has been manipulating events to frame Sir Charles Baskerville's death as an act of supernatural vengeance.