I've always been of the opinion that the difference between fantasy and horror was only a matter of perspective. The only way I am able to write fantasy is via horror. Considering that this will go in the extreme category I doubt that will pose much problem. I've got an idea but it's not completely fleshed out.
The setting is a slow moving world where one day lasts about one week of earth time. The dominant species of the world used to be a human like species called the (I'll think of something later, for now I'll call them humans). Then the dark ones appeared. The dark ones whom walk upright possess intelligence comparable to or greater then that of a human. Some could even pass for human. The seeders, ryders, and juggernauts, possess only rudimentary mental function. No two dark ones are exactly the same. This is due in part to the fact that while a few dark ones are born, most are hatched. Seeders and any other dark ones who feel the urge can plant their genetic material inside any living host. This host is then consumed after the young have hatched. The resulting creatures that spawn usually exhibit some of the characteristics of the host. The only thing that limits the growth and dominance of the dark ones is the sun. While sunlight does not kill a dark one it does impair them greatly. The heat makes them slow and sluggish. The light blinds and confuses their acute sense of vision. At first the dark ones got around this problem by hiding underground. But when the humans started to use this to their advantage a new solution had to be found. In one night the dark ones captured, killed, and enslaved half the population of the world. The half that faced away from the sun. Thus the chase began. All humans must constantly stay on the move. The trek across the planet is arduous and must be made constantly. Any human who falls behind and finds themselves facing the dusk will fall prey to the dark ones. Any dark one found in the day will be killed on sight. A lone woman found in the night by dark ones could look forward to a life of complete servitude at best. More likely she would be raped, beaten, eaten, tortured, and humiliated; in no particular order.
I'd like to add a bit more depth to the humans but I'm unsure as to how.You could play a human whose caravan fell behind. Or a soldier whose team ran out of time or fuel. Or something entirely different. What do you think?