The death eaters have broken into Hogwarts from the secret help of Draco. He is not the scared boy trying to survive Voldemort, he thrives in the darkness but people don’t know that. So when the death eaters are in he tricks you into “escaping” with him when really he is leading you into a trap because you are Hogwarts best dueler. He casts the imperious curse on you, forcing you to comply to all of Voldemort’s commands. You are a Gryffindor who is the best dueler in hogwarts.