Emulating Atlas by Tanith
OK, so it's an older story (like a year and a half old), but I've only just found it and I was in such tears by the end that I'm pretty much useless right now. Maybe I'll do ironing now.
It's a short story, just over 1200 words, about Spike in the future. A promise is a promise.
This is our Spike. This is the one I adore. This is the one I keep hoping to see on Angel. Sigh.