Final answer:
The word 'insufferable gloom' captures the essence of the opening effect in 'The Fall of the House of Usher,' creating an atmosphere of hopelessness that is underscored by the theme of mirror reflections throughout the story.
Step-by-step explanation:
The word that describes the single effect from the opening of Edgar Allan Poe's "The Fall of the House of Usher" is a sense of insufferable gloom. This mood is established as the narrator approaches the Usher estate and is overwhelmed by the bleak, oppressive atmosphere, which is in stark contrast with the outside world of hope and faith from which he comes.
Amidst this hopelessness and despair, the narrator's encounter with Roderick Usher and his morbid twin sister further intensifies the fatalistic theme that permeates the story. Additionally, Poe utilizes the concept of mirror reflections as a literary device to underscore the story's sense of despair and the inescapability of doom, both for the house and perhaps humanity at large.