What part seems unsolved? The girls are taken by their grandma, the ritual or whatever is completed. The girls are brainwashed by said grandmother and the "demon" doesn't return to their life until one of the women is ready to give offspring to a boy. The first boy in the blood line is what the demon wants to take in trade for... something, whatever that cult was about or whatever. That's my take on the story anyways, it seems very complete in my eyes. I will be super disappointed if they make a fourth. I would really like to see more movies that have this kind of cinematic feel but as far as this story goes I hope it's over.