The narrative essay, as the name indicates, is a way of sharing your personal experience. Since it is all about your personal experience, hence a narrative essay gives a better view about yourself as a person. However writing a narration text requires a lot of talent. It is completely different from a descriptive text in that the readers get to know about your experience and judge it according to them. In a descriptive essay, you need to give details about the topic at hand and also summarize it accordingly to present a justifiable conclusion.
Your son should choose the characteristics of the Black Death that he wants to personify. Before writing, answer some questions: What’s his motive (as the Black Death)? How does he feel about the “work” he is doing? Does he have a visible form or is he invisible? If visible, how do humans see him? Try to keep the personification consistent throughout. Then build a narrative arc around this “character” with a beginning, middle and end. Perhaps tell the story of visiting on a particular family. What does he learn from the experience? What is the point (thesis)?