Dark ominous mood, the struggle of light and Dark.
I have ideas for the cover scene, though don't limit yourself to these alone, you guys are the experts here, but this is just meant to give you some ideas...
The hero, with dark shaggy hair, dressed in bloodied tattered cloths, reminiscent of colonial days (He has two musket pistols and a magical tunic made of monster skin) He is a cross between Robert E Howard's Soloman Kane and Stephen King's The gunslinger. (just for your info, he gets severely bitten by a monster on his neck and shoulder, turns to black scars) His wife's corpse is draped in his arms, in a flowing dress or burial shroud (dress and shroud could be broken, tattered, flowing). She is blond, still hauntingly beautiful. Her eyes glow with a white mist (just for your info, there is and ancient entity possessing her corpse, but it is ultimately good)
He is standing with her in meadow at the boarder of the town (show his desperation) protected by light staring into a wall of shimmering, swirling darkness, there is an old stone wall in front of him, or perhaps he is standing in the darkness ( in the darkness there is a dark twisted forest where the trees are of prodgious size and more tentecled animal/fungus-like than plant) surrounded by it and monstrous shapes and silhouettes ((FYI, there are three hooded evil little girls with purple/pink eyes (they bit him, and work for the main villain(fallen angel vampire)), a humanoid beast monster, and indescribable cthulu-like abominations also lurk in the dark.))
The hero possesses a powerful cross necklace that he wields to emit purifying light that burns things in the darkness.
The darkness contains all things gothic horror, unconventional versions of vampires, werewolves, undead, ghosts, fallen angles, old gods, etc...
Other things that may help...
There is city of the dead in a swamp. The dead have glowing colored eyes.
There is a tower in town with an iridescent other worldly glowing sphere, keeps the darkness at bay.
There is a red rocky/glassy desolate wasteland with two fanged monoliths jutting up from the ground, lightning strikes them constantly, it is where he is trying to take his wife. Living fog stalk above craggy pits there. He could possibly stand before this place with his wife in his arms.
So hopefully i have given you some ideas to work with, I just need the cover to say dark fantasy/horror. It doesn't matter if your concept is literal or abstract, but it needs to be beautiful, and evident from the thumbnail size. Font needs to be indicative of the dark gritty theme, but not cheesy.