(Ch 6) What does the letter from Lanyon have in common with the disappearance of Dr. Jekyll?

When Lanyon tells Utterson of his illness, Utterson mentions that Jekyll is also ill. In response, Lanyon adamantly replies that he never wishes to speak of Jekyll again. Confused and shaken, Utterson returns home and writes Jekyll a letter, asking for an explanation for his mysterious behavior. Jekyll's reply, which arrives the next day, states, "I have brought on myself a punishment and a danger that I cannot name." One week later, Dr. Lanyon dies and leaves Mr. Utterson a letter with instructions only to read it following the death or disappearance of Dr. Jekyll. An honorable man who respects his friends' and client's wishes, Utterson does not open the letter.