At the beginning of the story the narrator describes his elderly "custodians" as "crouching and atavistic." He says also that there is, to his mind, "something inhuman in senility." The narrator compares one of those custodians, an old lady, to "a dead body, glaring into the fire with lack-lustre eyes." The elderly "custodians" are described in broadly negative terms. There is something slightly sinister or otherworldly about them. The narrator dismisses the warnings of these elderly "custodians" as the products of their senility. By the end of the story, however, we realize, as he does, that these warnings were accurate and should have been heeded. The point is that there is a wisdom in old age which the complacency of youth ridicules, and does not recognize until it is too late.
As the narrator approaches the red room, and remembers the stories he has heard about this room, he begins to feel a "twinge of apprehension." He acknowledges that the "sentinel shadows and watching darknesses" of the room "disturb" him, and he realizes, after seeing his white face in the mirror, that he is "in a state of considerable nervous tension." The narrator's fear grows and grows as the hour draws closer to midnight, when he expects the fabled ghost to make an appearance. Indeed, as he says, "the brooding expectation of the vigil weigh(s) heavily upon (him)." Thus the narrator's fear grows as much or more from the sense of dread inside his own mind as it does from any external cause. As his dread increases, so too his imagination becomes more excitable, and more prone to seeing sinister things in the shadows. The overall idea here is perhaps that fear is a product of our own minds.
Once inside the red room, the narrator's fear is so extreme, and his imagination so excited, that he begins to imagine a sinister supernatural force looming over him. This supernatural presence extinguishes all of the candles, as if with the sweep of "an invisible hand." It is then described as a "ponderous blackness," suggesting a weight that presses down upon and overpowers the narrator. The supernatural presence takes on no more concrete form than this. It is merely an impression of "blackness." This suggests that the supernatural is merely a product of the speaker's own excited, fearful imagination. It is an extension of the narrator's fear. Both his fear and the supernatural presence are thus manifestations of the inner workings of his own mind.