I was excited to see this story post as I just love it! I think you did a great job with your genre and prompts, and your characterizations were well done (Snape still stalking watching out for Harry and Harry getting fed up with all the machinations around him).
"During the ritual I will have to fuck you, Potter. You being so relaxed will make my task much easier – when I plunder your arse." I loved this line - it made me crack up laughing. Well done, teammate!