I didn't understand this sentence exactly. That is "By entombing Madeline, he creates that very "grim phantasm" with which he will struggle to the death--his prophecy becomes self-fulfilling. Why does his prophecy becomes self-fulfilling?
Lady Madeline is Roderick Usher's twin sister. She is sick with a disease called catalepsy. One with this disease is in a trance-like state with loss of voluntary motion and failure to react to stimuli. The narrator of the short story and Roderick assume that Madeline is dead (since she has a cataleptic shock) and proceed to entomb her for two weeks. Roderick seeks to keep the very creepy memory of his sister alive. This memory of his sister actually becomes real;Madeline later walks through Roderick's bedroom door, falls on her brother and kills him...they both die. Was Madeline really alive when they entombed her? WAs Madeline dead but the creepy house brought her to life? Really this is part of Poe's scary narrative. Poe is famous for self-fulfilling prophesies that are not completely explained.
