I think I just read the finished one but I'm not sure, an dI'll have to see if I can find it again. The synopsis is:
Someone drops a naked Buffy on Spikes door, and he takes her tortures her and repeatedly and very brutally rapes her. He really treats her like a dog in this fic, and she is so broken she develops Stockholm Syndrome, and Spike eventually falls for her. When Spike goes out one night he sees Buffy patrolling, and then it's revealed that this Buffy he's been torturing was sent by tptb. They wanted him to fall in love with her so that when he encounters the 'real' Buffy he will love her and help her. I know I really suck at summaries but that's what I recall of it and I'll see if I can find it for you. But Spike is a vamp, he is very evil, and Buffy is his slave who he treats with such cruelty it's kind of shocking.
Ok I found it it's
Reality Twists by Laure Alexander