Imagery has been vividly used in this excerpt. The light and dark have been used in symbolizing the good and evil things in that order Gandalf is the white that stands for good deeds. When Galadriel gives Frodo the Phial, this symbolizes the light that came during the darkest time.
The black rider stood for evils and everything that Mordor had covered with the blackness of shadowy. Each time Naqul passed near the overhead, the sun and moon became black.
