Final answer:
Dr. Hastie Lanyon witnesses Dr. Jekyll transforming into Mr. Hyde in the story, which is a pivotal and horrifying event.
Step-by-step explanation:
In the story 'Dr. Jekyll and Mr. Hyde', Dr. Hastie Lanyon witnesses the horrifying event of Jekyll transforming into Hyde. This transformation confirms Lanyon's worst fears about the dual nature of his associate and leads to his ultimate shock and demise.
The theme of respectable individuals embodying monstrous natures resonates with other Victorian Gothic literature, where characters like Dracula also reveal the darkness lurking beneath the veneer of societal respectability.