This is an AU fic where Spike(called william) was the one given the soul and comes to Sunnydale to help the slayer. He falls for Xander pretty quickly in the story. The story play out the different episodes of the show, but the hyena episode ends with the hyena not fully exorcized. He is now immortal. Xander eventually agrees to mate with william, which breaks the curse. William, now spike, uses the link they have due to the mating to influence Xander slowly to his side using his blood, that he forces xand to ingest to bring out his more amoral tendencies. The torture of mrs. Calander and spike leaving her in the library where Xander finds her as a present to kill is only one moment I remember. It ends with Xander sending spike through the achathla portal to hell and Xander feeling nothing from the bond. The author never started the next season, so it ended there. The main story is spike/xander love story from Xander's point of view.
I have been looking for the story for some time and can't remember where I read it. I know it was on its own site and found the link through another. It was a great read and would appreciate any help in finding it again.