When Fiona discovers she is half-angel, haf-demon, she is got up in the war between angels and demons over the human race and must choose a path to follow that will destroy or save the human race

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We don't need names in a logline. It adds nothing but an unnecessary word. We don't learn anything about a character from their name so it's far more important to give us a word that tells us something interesting.

Making a decision (save or destroy the human race) is not a good goal. Imagine watching someone decide something. All the action is happening inside their head and it could take 10 seconds or it could take a year. You need to give her a visual goal because film is a visual medium.

If she's half-angel/half-demon, what's at stake for her if the human race is destroyed? Has she been brought up as a human? Why her? Why is she the only one who can make this decision?

I feel like we need more specifics to really understand the story here and it would benefit from following the formula recommended on this site.