I'm writing a story based on a girl who when her mother was pregnant she was really ill and so she performed a black magic ritual to save herself and her baby… only the baby in return will bring around the end of the world as long as she is alive. does anybody have any ideas about what I could do to improve/complicate the story?
So probably the girl wants to live, or there would be no story… so give her a reason to. Does she have friends? Does she know what she will do? Does she try to save the world, or does she only care for herself? Is she likeable? Is she the kind of person you would expect to end the world? Try thinking about those sorts of things first. After that, I'd think about having someone who wants to kill her at any cost because of what she might do. Especially if she seems benign, this could cause confusion if she doesn't know about her origins. I don't know if that helps, but those are some basic things to think about. Even if you decide not to use any of those, I hope this helped!
Take a look at each of their motivations, specifically for living.
The mother obviously wants to live to take care of her daughter. The mother wants the daughter to live because, well, she created her and brought her into this world.
The daughter wants to live because.. why? @AllyM has some good ideas here.
You could work in some big twists in the daughter's motivations (especially since it sounds like she's the star of the story). What if.. she actually doesn't want to live? Her mother could do everything possible to make sure she stays alive and, while she eventually accomplishes her goal, it ultimately leads to the end of the world.