Final answer:
Dr. Lanyon in Chapter 6 of 'Dr. Jekyll and Mr. Hyde' becomes gravely ill and dies after witnessing Mr. Hyde's transformation into Dr. Jekyll, leaving behind a letter that explains the events leading to his death.
Step-by-step explanation:
In Dr. Jekyll and Mr. Hyde, Chapter 6, Dr. Hastie Lanyon experiences a great shock that leads to his deteriorating health and ultimately his death. This occurs after witnessing an unbelievable transformation of Mr. Hyde into Dr. Jekyll, revealing their true identity. The shock from this discovery is too much for Dr. Lanyon, and he becomes gravely ill, later leaving a letter for Mr. Utterson to read after his death which explains the events that led to his demise.