An irrepressible tremor gradually pervaded my frame; and, at length, there sat upon my very heart an incubus of utterly causeless alarm. Shaking this off with a gasp and a struggle, I uplifted myself upon the pillows, and, peering earnestly within the intense darkness of the chamber, hearkened – I know not why, except that an instinctive spirit prompted me – to certain low and indefinite sounds which came, through the pauses of the storm, at long intervals, I knew not whence. Overpowered by an intense sentiment of horror, unaccountable yet unendurable, I threw on my clothes with haste (for I felt that I should sleep no more during the night), and endeavored to arouse myself from the pitiable condition into which I had fallen, by pacing rapidly to and fro through the apartment. —“The Fall of the House of Usher,” Edgar Allan Poe Which detail in the passage changes the mood from unease to horror? “intense darkness” “a gasp” “an irrepressible tremor” “certain low and indefinite sounds”