Final answer:
The narrator assists Usher in entombing his sister, experiences the growing gloom of Usher's mental state, and witnesses the horrifying resuscitation and subsequent death of Madeline, which leaves him in shock.
Step-by-step explanation:
The narrator in Edgar Allan Poe's The Fall of the House of Usher is initially complacent, yet ultimately horrified, by Roderick Usher's announcement regarding his intentions to entomb his sister. The narrator aids Usher in the temporary entombment of Madeline in the family's tomb located beneath the house. This arrangement is described as being in a dark, oppressive atmosphere, exacerbating the ghastly and distressing nature of the act. As the narrative progresses, the narrator becomes more entangled in the emotional and psychological turmoil of his friend Usher, trying to provide comfort yet feeling the futility of his attempts against the pervasive gloom.
The true horror culminates when Madeline emerges alive from her tomb, resulting in the death of both siblings and the collapse of Usher's mental state, as well as the literal fall of the House of Usher. This stark and grim resolution leaves the narrator shocked and emphasizes the themes of death, entombment, and the supernatural that are prevalent in Poe's works.