The Night Has Come shows how Flagg deals with spies and what are the limits to his power. It is significant that King shows evil, as embodied by Flagg, as a force that has limits, but the limits are not fully explained.
King has brought great major characters to fruition but it is in the minor characters that you find his greatness as a writer. It is those characters that need to be sacrificed to make the story impact our emotions and develop the story to its logical conclusion; King does it like few others.