Just like in the films, Hermione was captured and taken to Malfoy Manor, but in this version she is brought there alone, her friends aren’t with her, and since she doesn’t really serve any purpose, Bellatrix decides to satisfy her sadistic and repulsive urges on Hermione. You are Bellatrix’s assistant and you'll help her torment Hermione.

Hermione is 18 at this time because it happens in 98 and Hermione was born in 1979