I’ve seen Bone Tomahawk thrown out there but let’s give some love to the father: The Proposition. It’s so gritty and raw you can feel the sweat dripping off the characters, and the plot is just downright dark. It’s got some disturbing sequences and so I would absolutely put it in the realm of horror, but it’s a wonderful dark western too. Guy Pearce is amazing. It’s available in high quality in full on YouTube - definitely check it out.