I read this fic so long ago, I can't clearly remember the details. The main element of the plot was a prophecy concerning three people: Dawn as the Key, and Buffy and Spike as her protectors, though each had a particular role. The role they had was connected to some special objects. Spike had a mystical sword, and Buffy was supposed to wear an elaborate piece of jewelry sort of like a necklace, but that Spike would be able to bite her neck while she was wearing it. I forget Dawn's object.
Buffy and co. ended up going to LA for help looking for answers with Angel's gang. At some point in the Hyperion, during an attack Spike is taken over by a need to protect Dawn and turns into a killing machine. He is reacting on a pretty primal level, and when something sets him off, he grabs Willow. I think using the sword might have had to do with it. Anyway, he's latched onto Willow because he barely recognizes her, and everything in him is shouting to feed from her, but he manages to stop himself. Barely. He can't make himself let Willow go, so the others are trying to figure out what's happened and how to stop it while he stands there holding onto her. He keeps repeating himself, over and over, not realizing at first that he's saying it out loud, "Family, not food. Family, not food" to remind himself why he mustn't hurt her. | 
