There was an iciness, a sinking, a sickening of the heart (1)
Look at how Poe uses alliteration in this sentence to set a rhythmic, spooky mood. Alliteration means using words in succession that begin with the same sound.
There can be no doubt that the consciousness of the rapid increase of my superstition—for why should I not so term it?--served mainly to accelerate the increase itself. Such, I have long known, is the paradoxical law of all sentiments having terror as a basis. (4)
The same is later true of Roderick’s fear…
While the objects around me--while the carvings of the ceilings, the sombre tapestries of the walls, the ebon blackness of the floors, and the phantasmagoric armorial trophies which rattled as I strode, were but matters to which, or to such as which, I had been accustomed from my infancy--while I hesitated not to acknowledge how familiar was all this--I still wondered to find how unfamiliar were the fancies which ordinary images were stirring up. (6)
Fear has the ability to alter our perception of even the most ordinary of objects.
An air of stern, deep, and irredeemable gloom hung over and pervaded all. (7)
This air of gloom is akin to the actual fog that surrounds the mansion.
To an anomalous species of terror I found him a bounden slave. "I shall perish," said he, "I must perish in this deplorable folly. Thus, thus, and not otherwise, shall I be lost. I dread the events of the future, not in themselves, but in their results. […] I have, indeed, no abhorrence of danger, except in its absolute effect--in terror. In this unnerved—in this pitiable condition--I feel that the period will sooner or later arrive when I must abandon life and reason together, in some struggle with the grim phantasm, FEAR." (11)
Roderick isn’t afraid of death or pain; he is afraid of fear. And as he predicts, this is precisely what he dies of, when Madeline comes back from her tomb and scares him to death.
He was enchained by certain superstitious impressions in regard to the dwelling which he tenanted, and whence, for many years, he had never ventured forth--in regard to an influence whose supposititious force was conveyed in terms too shadowy here to be re-stated (12)
Over and over again, the narrator reiterates the impossibility of his accurately conveying the events of this episode on the written page. The story is too spooky, too other-worldly, too scary for him to get across in full.
While he spoke, the lady Madeline (for so was she called) passed slowly through a remote portion of the apartment, and, without having noticed my presence, disappeared. I regarded her with an utter astonishment not unmingled with dread--and yet I found it impossible to account for such feelings. A sensation of stupor oppressed me, as my eyes followed her retreating steps. (13)
This line is great evidence for the argument that Madeline is nearly a ghost or apparition.
I learned that the glimpse I had obtained of her person would thus probably be the last I should obtain--that the lady, at least while living, would be seen by me no more. (14)
Look at the foreshadowing in this line – there is such a sense of doom that we can predict Madeline’s death quite easily.
Certain accessory points of the design served well to convey the idea that this excavation lay at a exceeding depth below the surface of the earth. No outlet was observed in any portion of its vast extent, and no torch, or other artificial source of light was discernible; yet a flood of intense rays rolled throughout, and bathed the whole in a ghastly and inappropriate splendour. (17)
Pay attention to the number of instances of eerie light, or light of unknown origin, that crop up in this story…