It’s a Gothic standard to contrast "the light" and "the darkness" or "the shadow." The whole work is supposed to seem "dark," foreboding, threatening, and mysterious. This usually means that the darkness is what prevails, and the light is either weak and isolated or completely absorbed. "Red Room" is no exception; it’s chock full of this stuff. You want "overwhelming darkness"? It doesn’t get much better than this:
And looking around that large sombre room, with its shadowy window bays, its recesses and alcoves, one could well understand the legends that had sprouted in its black corners, its germinating darkness. My candle was a little tongue of light in its vastness, that failed to pierce the opposite end of the room, and left an ocean of mystery and suggestion beyond its island of light. (31)
The whole house, and particularly the red room, is dark. This darkness threatens the narrator, because he doesn’t know what might be lurking in it. It suggests dangers to him that aren’t really there. In the dark, the narrator first thinks the statue of Ganymede and the Eagle in the hallway is "someone crouching to waylay" him (29). And then of course there are the shadows, which have "that undefinable quality of a presence, that odd suggestion of a lurking, living thing" (33). The darkness that inspires fear, and the "Fear" itself are intimately connected.
The narrator’s own internal struggle against his fear is mirrored in his physical struggle with the darkness of the room. As a result, you can see Fear/Darkness and Reason/Light as closely connected pairs. By filling the red room with candlelight and illuminating its dark recesses (particularly the alcove), the narrator gives himself a sense of security and keeps his fear at bay. Reason, like light, is something that reveals. Reason is supposed to eliminate darkness or obscurity, to show that what’s really there is not scary. When the candles begin to go out, the narrator engages in a literal fight against darkness as he tries to keep the room lit. As he becomes overwhelmed by darkness, the narrator grows increasingly frightened and loses his self-control. When the light is completely gone, so too are "the last vestiges of reason:"
…darkness closed upon me like the shutting of an eye, wrapped about me in a stifling embrace, sealed my vision, and crushed the last vestiges of reason from my brain. (45)
Fittingly, the story ends in the daytime, when the darkness is gone and the narrator can come back to his senses. In the light of day, he can realize what he was really fighting in the room.