Starts in School Hard, where Spike gets the upper-hand in fighting and goes to bite Buffy, but sees the Master's bite, gets angry, and scratches it.
Not sure if same story, but on Halloween, Buffy and Spike are in a church and do a binding ceremony with a cord. Angel, Soldier!Xander, Cordelia, and Ghost!Willow are trying to get in to find Buffy, but their path is blocked by the kids-turned-demons.

This sounded so interesting. When I read it last night I knew that it was the one you were looking for.
it's
Duty or Destiny by Tasha
http://www.bloodshedverse.com/stories.php?go=read&no=949