depending on the character choice, we can to the astronaught on a mission who mistakenly brings back an alien
species. which is after her once she gets to earth. 

the demon idea is the war between heaven, and hell has torn the fabric of space and time, allowing the ancient
evil onto the world. as the angel or human / hybrid depending on heroine's choice of species will be the last stand, but 
she will be alone until the final confrontation. 

the back story is more of a survival style, while the main point of it
is not as much story as it is 18+ style struggling to survive, and deal with the enemy, or enemies depending on which kind of char
the heroine decides to use.