Much of the fear addressed in "The Fall of the House of Usher" is related to decay and death. As the narrator arrives, he contrasts the long-standing, enduring trees with the decayed aspect of the house. Usher appears extremely pale, and the impending death of Madeline dominates the atmosphere in the house and has caused Roderick to lose his mind. The cataleptic condition of Madeline also brings with it repeated death-like experiences, and the fear of a premature burial, another of Poe's topics.
You can follow this trend of thought and illustrate it with those elements and passages in the story that relate to this decay, with its accompanying gloom, and with all those that refer to death and to untimely entombment.
