Final answer:
The question seems to be about Arthur Conan Doyle's 'The Hound of the Baskervilles,' where Stapleton reveals to Watson that he is a member of the Baskerville family, a revelation that is both shocking and indicative of his malicious intent.
Step-by-step explanation:
The passage provided is not from the story where Stapleton shocks Watson, but seems to be from Nathaniel Hawthorne's short story 'The Birth-Mark.' Therefore, it does not contain the information needed to answer the question regarding Stapleton's revelation to Watson.
The question appears to be related to Arthur Conan Doyle's 'The Hound of the Baskervilles.' In that novel, the shocking revelation made by Stapleton to Dr. Watson is that Stapleton is actually a member of the Baskerville family, which is a significant plot twist because he had been masquerading as just a naturalist living on the moor.
It is shocking because Stapleton's intentions towards the other Baskervilles and the motive behind his actions are malicious.