While it doesn't have to be decided upon first, a title will be a key ingredient in determining who will watch your work. You must come up with title that suites the genre and appeals to fans. One word, ambiguous titles have often worked well to draw theater goers to fantasy horror movies despite the ratings. Names like Signs, Colossus, and Blade offer a powerful word without any attachment, which will alway pique an audience's interest.
Determine the details, location and characters for your movie. Where will your fantasy horror take place? The most exciting part of creating a Fantasy Horror is that you can take every aspect of history you love and combine it all together. Some of the best fantasy movies are based on topics like genocide, war, and technological advancement. Use what interests you to create your environment and use characters that are out of their element in that environment or at least overwhelmed. If the protagonist must find his way in a strange world, you have already created the conflict of the story.
Determine what the inciting incident will be. You will need to create a world that is ripe to crumble or boil over with atrocity. Then, within the first 15 minutes of the movie, you need to address the big problem that occurs. Most fantasy horror movies move from a state of order, to chaos, and then the rest of the film is devoted to restoring order.
The terror must persist. At the end of the film, leave your audience afraid. Bringing the antagonist back has become a tired cliche. But you still want to leave everyone feeling uneasy. If everything is wrapped up nicely, you leave no room for a sequel.