Blood Traitor (Severus Snape's Daughter)

At the age of six, Sofianna Snape witnessed the murder of her beloved mother. She was then kidnapped, leaving Severus cold and lonely. Three years later, Sofianna is broken (quite forcefully) from her captor and has a choice: leave the wizarding world and attempt at a normal life as a muggle, or seek out her father and go to Hogwarts. She chooses the latter, and because of her skill and knowledge, is placed in first year early, two years behind Harry (set in POA).