Spike goes off after the bathroom to get back his soul, or so he thinks. He learns that he's really come for something else. He goes through these trials and becomes a different type of vamp, a cousin that does not prey on humanity. He forms a blood-brother type bond with (Khai?) and they are each supposed to teach one another about "The Light & The Dark" respectively. Khai starts dating Willow, and has a half sister that is a really powerful witch that come back from England with Giles and they start dating.
The guy whose position in the vamp cousin clan Spike is supposed to take is called the "Keeper of Truth" I think, and Spike is the only other vamp to go through the trials since him, a 1000 years.
I remember reading it around the time the show ended and it was already a completed story. | 
