I remember Xander is a highlander style immortal, I think this was second story in a series the first was a Spike/Methos fic with mention of Xander becoming immortal. But the one I'm thinking of is were Dawn notices Spike's missing and no one but Xander will believe her. So Xander looks for Spike and finds him near death with no fangs and wooden balls for eyes. Xander knows Spike could drain him over and over and it wouldn't matter..but what he didn't realizes is that he re-sired spike. It was Angel who told him, also spike reverts back to William and it takes a while for his memory to come back. Then they went after the people who did that to spike..I remember spike called the one who tortured him the Doctor.
That's it if anyone could help I'd really appreciate, thank you :)