I keep seeing people recommend Sarah J. Maas for this, and while she's fun, her fae are more about the glittering courts and power dynamics. If you want a truly chilling, morally gray take, you need to dig into the indie and older shelves. Holly Black's 'The Folk of the Air' series nails that vibe—the fae are cruel, clever, and the protagonist has to become just as vicious to survive. It's not just dark because of bloodshed, but because of the psychological games. The line between ally and enemy is constantly blurred, and the 'love' story feels more like a mutual obsession that leaves you unsettled.
For something that leans into body horror and unsettling transformation, Emma Hamm's 'Hollow' is a lesser-known pick. The fae realm is a rotting, beautiful corpse, and the human protagonist's change is portrayed as a loss of self that's both terrifying and alluring. It's less about epic battles and more about the slow erosion of humanity, which I find way more effective for dark fantasy. That sense of dread, of being consumed by something ancient and indifferent, stays with you long after the last page.