A few observations about dark fantasy tropes after reading too much of it:
The genre's central trick is moral contamination. In epic fantasy, the hero picks up the sword and stays clean. In dark fantasy, the hero picks up the sword and something in them changes. The weapon costs something. The magic costs something. The victory costs something. Readers stay because they want to see what the bill looks like.
Anna Smith Spark's The Court of Broken Knives reads like someone translated a battle scene into poetry and then set the poem on fire. Her prose is fragmented, almost liturgical, and the violence doesn't serve the plot so much as the plot serves the violence. It shouldn't work. It works anyway.
The chosen one trope shows up in dark fantasy too, but the twist is that being chosen is the punishment. The prophecy is a curse. The gift is a disease. The narrative promise of specialness gets turned inside out, and the character spends the book trying to survive the thing that was supposed to save everyone else.
Dark fantasy writing tends to attract prose stylists. I think that's because bleakness requires a certain musicality to sustain. You can't write 400 pages of suffering in flat, workmanlike sentences. The beauty of the language becomes the counterweight to the ugliness of the content. One needs the other.
Glen Cook figured this out decades ago with the Black Company books. He kept the prose deliberately plain, almost journalistic, and let the horror live in the gaps between what the narrator reported and what actually happened. A different kind of music, but music all the same.
Revenge is the most reliable engine in dark fantasy. Evan Winter's The Rage of Dragons runs entirely on Tau's need to destroy the people who took everything from him. The book barely slows down for 500 pages. What makes it interesting as a piece of dark fantasy writing is that Tau's single-mindedness is both his greatest weapon and the thing hollowing him out, and Winter doesn't pretend those are separate problems.
The anti-hero in dark fantasy works differently than in, say, crime fiction. In crime fiction, the anti-hero is usually competent and morally flexible. In dark fantasy, the anti-hero is often broken in a specific way that the world's magic or politics has made worse. Their flaw isn't charm or ruthlessness. Their flaw is that they let the world reshape them, and they can feel it happening, and they keep going.
I'm genuinely unsure whether dark fantasy needs a redemption arc to work, or whether the expectation of redemption is something readers import from other genres. Some of the best books in the space offer no redemption at all. The character ends worse than they started, and the book is honest about that, and somehow you feel like you read something true.