It is key to note that Book IX signals a change in the tone of this epic classic, as Milton turns to the original sin of man and Adam and Eve's disobedience in the Garden of Eden when they choose to eat from the Tree of the Knowledge of Good and Evil. Note how Milton indicates this change in tone:
I now must charge
Those notes to tragic; foul distrust, and breach
Disloyal on the part of man, revolt,
Stylistically, to accompany this much darker and more serious purpose, you might like to analyse the way in which images of light and darkness parallel the contrast between good and evil in this section. For example, note the way that the action opens at midnight and we are told that Satan has just spent the last seven days hiding in night. As he disguises himself as a snake, note how the speaker describes Satan:
Through each thicket dank or dry,
Like a black mist low creeping, he held on
His midnight search.
Note the way that diction in this quote helps convey the darkness and evil of Satan as he is compared to a "black mist" that "creeps" and conducts his "midnight search."
Note how this is juxtaposed by the dawn and the rising of the sun and its "sacred light." A scene of goodness is described to accompany the advent of light and the arrival of morning:
Now when as sacred light began to dawn
In Eden on the humid flow'rs, that breathed
Their morning incense, when all things that breathe,
From th'earth's great altar send up silent praise
To the Creator, and his nostrils fill
With grateful smell...
It is clear that images of dark and light correspond with good and evil, which of course becomes incredibly important in this Book which deals with such an important and weighty topic as the birthing of original sin.