If you haven't read The Playroom by calanthe_fics from last year's hp_darkfest, I'd recommend that. Bellatrix is pretty horrible to Draco. No mpreg, but definite cross-dressing. Not a slavefic, but it's disgusting and amazing. http://community.livejournal.com/hp_darkfest/2731.html

Otherwise the WIP Sectumsempra Animi by Sakaim is fuhreaking fantastic. I got completely addicted to it and then realized there was nothing after Ch. 6. It's amazing, but read at your own risk, because you will want more by the end: http://archive.skyehawke.com/story.php?no=14849