Delphi Diggory and the Secrets of Slytherin

After the defeat of Lord Voldemort, Cedric Diggory's parents find and adopt Lord Voldemort's own daughter, Delphi. Being loving and supportive parents, they give Delphi a wonderful childhood so that she grows into a kind and wonderful young woman. There's just one problem: the Diggorys never really got around to telling Delphi that she's the heir to Slytherin...