Is it
Rebellion by Thianna
Summary: *COMPLETE* AU - Buffy just moved to Sunnydale with her mother to attend college. She knows nothing of demons or vampires until she meets Angelus. Angelus is Master of the Aurelius coven in Sunnydale and has decided to keep Buffy as his new plaything. Spike's bored and wants to leave Sunnyhell but his decision has some consequences.
I haven't read this in a while, but this is the only human Buffy as Angelus's slave story that I remember. But she doesn't stay human throughout the story. If I remember correctly she does eventually turn into the Slayer. But it is after Angelus captures her and uses her to humiliate Spike and they leave the mansion. I also think the Master is in it towards the end, and Spike kills him because he wanted him to kill Buffy. But, beyond those extra plot points, I'm about 95% sure this is the story you are looking for. Unless there is some other human Buffy as Angelus's slave out there that I don't know of, which I would love to read if there is.
Melts
