[…] I stood with the candle held aloft, surveying the scene of my vigil, the great red room of Lorraine Castle, in which the young duke had died. Or, rather, in which he had begun his dying, for he had opened the door and fallen headlong down the steps I had just ascended. That had been the end of his vigil, of his gallant attempt to conquer the ghostly tradition of the place. (31)
This passage makes the young duke out as something of a hero: he’s "gallant." He was trying to fight against darkness, superstition, and the "ghostly tradition" of the castle. In other words, he was trying to be the good guy and do battle with the forces of darkness. But he died. Whatever it is that’s in the red room remains.
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)
This is the most suggestive piece of light/darkness imagery in the story. It conveys the sense that the darkness is something overwhelming, and the light just a small blip in the midst of it. It’s literal in this case, but there’s definitely something symbolic about it too. The narrator, with his candle, is the bearer of light in the midst of an enormous darkness. Feels rather threatening.
The shadow in the alcove at the end in particular, had that undefinable quality of a presence, that odd suggestion of a lurking, living thing, that comes so easily in silence and solitude. At last, to reassure myself, I walked with a candle into it, and satisfied myself that there was nothing tangible there. I stood that candle upon the floor of the alcove, and left it in that position. (33)
The darkness in the red room threatens the narrator and feels alive. It "lurks." He tries to fight it off by combating it with light, in the form of the candle. At this point, the literal struggle between light and dark in the red room begins (and with it, the more suggestive struggle between the narrator and some "dark power").
These I put in various knick-knacks of china with which the room was sparsely adorned, lit and placed where the shadows had lain deepest, some on the floor, some in the window recesses, until at last my seventeen candles were so arranged that not an inch of the room but had the direct light of at least one of them. It occurred to me that when the ghost came, I could warn him not to trip over them. The room was know quite brightly illuminated. There was something very cheery and reassuring in these little streaming flames, and snuffing them gave me an occupation, and afforded a helpful sense of the passage of time. (35)
For a while, all is well. The narrator appears to have satisfactorily conquered the darkness of the red room – and his own fear – by filling the place with candles. The light is "cheery and reassuring."
Then something happened in the alcove. I did not see the candle go out, I simply turned and saw that the darkness was there, as one might start and see the unexpected presence of a stranger. The black shadow had sprung back to its place. (37)
When the "black shadow" springs into place, it’s as if the force of darkness that was originally in the red room has reasserted itself. It suggestively reappears in exactly the spot that had most disturbed the narrator originally: the alcove opposite the fireplace. You might call that spot the "locus of darkness." That’s where the darkness "comes from," and where it seems strongest.
While I stood gaping, the candle at the foot of the bed went out, and the shadows seemed to take another step towards me. (39)
The darkness, in the form of the shadows, advances. The language here really makes it sound as if a battle is ensuing between the narrator and some living, evil force that’s coming toward him.
[…] but for all that the steady process of extinction went on, and the shadows I feared and fought against returned, and crept in upon me, first a step gained on this side of me and then on that. I was now almost frantic with the horror of the coming darkness, and my self-possession deserted me. (43)
This time the image of a "battle" between the narrator with his light and the advancing shadows is made explicit with the word "fought." The shadows are the narrator’s enemy. He fears them, and they are gaining on him. The image of a storm cloud sweeping out the stars is particularly powerful.
I leaped panting from candle to candle, in a vain struggle against that remorseless advance. (43)
Here again we see language suggestive of a battle, a "remorseless advance" of the enemy. Now it definitely looks as if the narrator is losing.
I flung out my arms in a vain effort to thrust that ponderous blackness away from me, and, lifting up my voice, screamed with all my might – once, twice, thrice. (45)
The narrator has lost. The darkness has eliminated all light, overwhelmed him, and now surrounds him completely. His wits are gone. The image of the screaming narrator flinging his arms around conveys complete helplessness.
"I knew that was it. A power of darkness. To put such a curse upon a home! It lurks there always. (60)
The man with the shades, unlike the narrator, calls fear a "power of darkness." Although fear was already personified in the narrator’s description, the old man’s use of the supernatural-sounding (and very Gothic) phrase makes it even clearer that fear is an active evil. It also brings out the tight connection between the fear the narrator was feeling and the literal "darkness" against which he fought in the red room. This is a great wrap up for the story.