Spander Search

'I am looking for a Spander fic I read once, cant remember where though.
I think in it Spike and Xander are sort of dating, but Xander thinks that Spike is only with him cos Buffy doesnt want him.
They are out one night and they are attacked by a demon, Xander calls out for help but no one helps and he sees Spike rushing to help Buffy, which increases his believe that Spike doesnt actually want him. It turns out that the Demon has made Xander invisible or something.
I believe Xander ends up fatally injured and has to be turned.
If anyone can tell me what this is, it would be most appreciated'

Thanks for any help.