Is there a myth for this?

If you're writing a book....why do you need a real myth to expand on? Even if there is a real myth out there that is exactly what you want, whatever you write about it will be false since you didn't create the myth or know the basis for it.

If you can write a book, you can write your own myths.

You can always do a search online or in the library of world myths pertaining to a women born with powers that will end evil.

Please keep in mind that this idea has been done in many ways and many mediums. Make sure yours is unique and original.

lore says that a child that is half demon can possess strong powers and are more powerful than demons. Problem is, demons don't exist.

Sorry, no such myth exists.