You ever notice how a lot of dark fantasy covers seem to use the same visual shorthand? The broken crown, the cracked skull, the barren tree against a blood-red sky. It works, sure, but it gets a bit predictable after a while. I think the most effective ones are quieter. There's this one edition of 'The Black Company' that's just a tattered standard on a pole in a muddy field, the fabric so worn you can barely make out the emblem. No monsters, no fire. Just desolation and the weight of a long, ugly war. It tells you everything about the tone before you even open it.
Another angle is when the cover art subverts a classic fantasy trope. A pristine, heroic portrait of a knight... but his reflection in his shield shows a monstrous, twisted face. Or a beautiful, intricate floral pattern that, when you look closer, is made of thorny vines choking a skeletal bird. That sense of corruption, of something beautiful gone deeply wrong, is the heart of dark fantasy for me. It's not about how much gore you can splash on the front; it's about suggesting a world where the foundations themselves are rotten.