Light vs darkness is not and never had been the only theme in novels. In fact, in the simplistic sense in which it is practiced today in things like post-Tolkien fantasy novels, it is a pretty new phenomena.
Take most fairy tales, for instance, have more to do with the moral fortitude to resist the glamour of evil than they have to do with pitched battles of your wand vs. my wand.
But if resisting the glamour of evil is a strong recurrent theme, the failure to resist it is a necessary contrapuntal theme. Thus we make two masks, comedy and tragedy. And the tragic figure's downfall lies always in that failure of moral restraint.
Every heroic figure at some point is tempted from, and often strays from the path of light, only to return to it. Every tragic figure falls into the path of darkness and fails to return from it.
Light into darkness into light is a story.
Light into darkness is a story.
Darkness into light is a story.
Darkness into light into darkness is a story.
There are stories in which the protagonist stays perpetually in the light, though they seem to involve some secondary character being assisted from darkness into light. Perhaps there are stories of a perpetually dark protagonist leading a secondary character from light into darkness.
But it does seem to me that changes of state, and not a mere succession of incidents, are necessary for a story. It is one of the most frequently suggested elements of scene-building that the character should be in a different state at the end of the scene compared to the beginning. The same is surely true of stories as a whole.
If this all holds together, then the source of interest in stories is the nature of the transitions that take place. Stories are experiences. They deal in the question, what is it like to...? If the essence of tension is that we want to know what happens next, it is because we want to know what the thing that happens next is going to feel like. We don't just want to know that the bride and groom got married, we want to attend the wedding. That is the difference between the marriage notices in the paper and a romance novel.
So, yours is a light into darkness story. Your task in telling this story isn't any different from telling any other story. What is it like to transition from light into darkness? What is its texture? What are its emotions?
The challenge is, do you understand this path well enough to tell us what it is like, or at least to convince us that this is what it feels like? It isn't something that you can solve with some writing trick. It is a vision problem, not a writing problem.
There are lots of scenarios that we can conceive of. But there are only so many stories we can actually tell convincingly, because having a convincing vision of an experience is a different and more difficult thing than merely imagining that a situation could occur. Have you an eye that can see in the darkness? If so, write you story as you would write any other story and let your vision come to light.
Edit to address comments
The character who starts in the light, is tempted, falls, and then instead of turning to the light once more, delves deeper into the temptation, the darkness. The reader starts out sympathetic, certainly, but it's that conscious decision to go deeper into the darkness (and subsequent similar decisions) which I feel will push the reader away.
I think the issue here is that it implies a story that goes on past the end of the arc. I have often argued that the structure of a story is essentially moral. The protagonist is forced to face a choice between values, and makes a fundamental decision. (This is analogous at least to ideas like the mirror moment or the inmost cave.) The rest of the story from that point simply demonstrates that they have really made this choice. The choice itself is the great inflection point of the story.
Once that decision has been made and demonstrated, the story is over. They lived happily ever after. There is no more left to say.
So the moment of choice in a tragic story is that moment at which "instead of turning to the light once more, [the character] delves deeper into the temptation". After that moment of choice, "that conscious decision to go deeper into the darkness" what remains to do? Only to demonstrate that the choice has been made.
I suspect that the demonstration of the intent to turn toward the light takes a lot longer to tell than the demonstration of the intent to turn toward the darkness, which would give tragedies a different structure from comedies.
But once that decision has been made, and we have made up our minds about them, our interest in them is over. Prince Charming may be a perfectly nice guy, but we don't want to go on reading about him driving the kids to soccer practice and remembering to bring Snow White flowers on Valentines day. Story is over. Move on.
Similarly, while the dark protagonists choice is still before him, we are interested. Will he turn to the light or continue into darkness? Ah, he continued into darkness. He did not live happily ever after, he burned in hell for evermore. Story over. We don't want to read about him getting up from his bed of nails every morning and bathing in the lake of acid full of piranhas. He made his choice. His story is done. Next.
It is not the darkness of those subsequent decisions and actions that will push the reader away, but their banality and predictability. Once the decision between light and darkness is made and demonstrated, the story is over. The protagonist has been redeemed or damned, and there's the end of it. There is nothing more to see.