I've been thinking about it for the past few weeks now and I still don't really know what to do about it. I have a few characters fleshed out and I've got a main character and all. I just don't know what to do about the plot. What I have right now is the idea that there was this war previously between witches and demons. This one witch, who is also half demon, defected to the demon side and led the demons to victory. Now most of the witches are dead, as is most of the human race. The head witch, who survived by abandoning her friends, is informed about this guy who also happens to be a witch-demon. She finds him, and a few others and then heads out to go and fight the demons. Good? Bad? Let me know.
Let me ask you a different question: What is making you want to write this story? You've got your actors, you have a wardrobe department, you've got some sets and some scenery. What you don't have is a script. With all of these other elements already in place, some time and investment has already been devoted, so what are you trying to say with all of this. What's the why of your story?
Isn't he great? That was such a precise comment, we need a new reward category, (if we could get the sci-fi short story contest moving that is). I'm motivated to write because I have something to say, and I want to write the protagonist I wish other people would write but they haven't. As I began the story in my head, and still as I continue to write, revise, and edit, I keep that thing I want to write about as an anchor for the story. It's come into focus while before it was more vague but it remains a framework the story grows around.
I agree with @Wreybies about the why. The basic premise you have going is fine - you can make a perfectly good story of it. When it comes to why, think in terms of your character. Why is she going back out to fight demons? What are the stakes for her (besides that the demons might just want her dead). As you flesh out her goals, think of what roadblocks exist to those goals and how you can throw them at her. Then the beginnings of a story will start to form.
To me, the obvious "plot" is to have a "worthy opponent." If your MC is carrying your message, the opponent would embody the antithesis. The struggle between your message and the anti-message can be played out any way you like.
I always struggle between wanting to create characters and then just follow them around and see what they get up to (i.e. character-driven) and having a set plot in place before I let my characters loose (i.e. plot-driven). But I think in the end you have to have some idea of plot or else it just becomes a character study, not a book, you know? [link removed]
There's your first problem. You're starting with the character, defining him/her, and then looking for a situation. That's backward. A given plot requires a character tailored to the needs of that plot. If you end up needing a scientist it does you no good to have created an athlete. And given the number of possible plots, the odds say you wasted the time to define him. It sounds as if what you're really missing is a good knowledge of the process of creating a story for the printed word, and the elements that make up scene and story. There's a lot to writing fiction that's not obvious, and the skills we learned in our schooling aren't the professional tools needed—though they don't tell us that. So if you truly would like to write fiction that will excite tyour readers you need to do exactly what you would do were you to want to play a sport or learn any other profession, and that's to dig into the basics to get the answers to questions like the one you asked, and the ones you don't yet know to ask. And for that, the fiction writing section of your local library system can be a great resource.
I'll just pop in and add that ideas are cheap. It's incredibly easy to get a basic idea, premise, or plot in your head. Stories are everywhere really and even terrible ideas (like doing a mixture of Oliver Twist and Pokemon) can work out very well if the skill of the writer is adequate. So yea, don't be afraid to discard this idea if you're uncertain if it'll work. Write the story that you're so eager to write that you can't help yourself but to write it.
Better yet, I think you should write about two stoners trying to plot a novel. "So, like, there's this underworld war, right? and there's this demon- witch chick whose totally rad, man, one sec she's fighting for the demons then is like BAM, forget you dudes I'm fighting for the witches." "Woah."
Strip them down a bit, the fact the you have characters proves an embryo of a plot since you are putting them somewhere, but the fully fleshed out bulk of their back story might be interfering with the plot development. Don't be too precious about them, change them if needed. You have a back story so where do you want to go next? Its a fine balancing act we all struggle with. Try think outside the box and start looking at what you want to say. Is it a love/action/redemption/adventure/battle driven story or none the above. Then how would your characters fit into it. If the characters don't fit evolve them. One the ball is rolling you have too many plot points to contain Wish we could say do this..... but then it wouldn't be your story to tell, just a collaboration of the forum. And there are some crazy people here hahahah JOKE!!!!
By this logic, a series with the same central characters is doomed to failure. Clearly, you are taking too narrow a view of the writing process, because there are writers who write this way on a daily basis. Every writer has a process of his or her own, Some writers begin best with a set of central characters, others begin with a situation and build characters to best carry it to a conclusions, and still others develop both incrementally. There is more than one way to skin a cat, or to build a cat from feline parts. It's naive to speak of the process of creating a story. What you really mean is your process.
Don't be silly. After the first one the plot type is defined, so the original character will fit. If the story is about a hard boiled detective you can be certain the second one won't require him to be an expert on religion, or a scientist, or.... Fully define your protagonist before you even know what the story is about and you've limited your options—and quite possible wasted the time it took to type it. Every successful writer might. But the 999 out of 1,000 new writers who are rejected certainly don't. But even then you're making the unsupportable assumption that any approach is valid and that there are no common elements that all writers will share. It's a profession, remember? They teach the skill in university majors that are anything but easy to get through. Do you really think they just take a class full of people and say, "Do your thing because any approach is valid?" No one can teach you talent. But they canbtrain it, just as in any other profession. And they can teach you how to use the tools of the professional, just like they do any profession. “Self-expression without craft is for toddlers.” ~Rosanne Cash
There's more than one road to the Big Apple. The same is true for success in writing. By Gods, people sometimes accuse me of being too rigid in my recommendations. In comparison with you, I'm a reckless maverick.