Lucius/Harry Ds fic
It started 5th or 6th year, i think, Harry's wandering the halls at night and catches Lucius leaving Snape's office. They end up in an empty classroom and Lucius doms Harry into sex. Harry already knew he was into the scene and Bill and Fleur know so he turns to them for support. Lucius surprisingly presents Harry with a relationship contract and that's as far as i got. I know it was alot longer on AO3.