I might be mixing the fics here but I just went back and re read winters' storm and thats not it.
I think Buffy is undercover in spike’s home/mansion/compound? I don't really remember how she's involved. Maybe she is a prisoner i have no idea. Spike is a crime lord/boss/criminal whatever.
I think however buffy is there on purpose investigating high level black market art theft. And maybe theres backup in the house. like xander? Or she finds out after the fact that xander is part of a team fighting spike as well? Maybe with one other person. possibly gunn.
SPOILERS buffy wins his trust in the end and he shows her his room of perfectly forged paintings by the masters all actually painte by him. I think he had some kind of deal cut the whole time where he was desperately trying to get out of that life. Maybe not.
Honestly this memory is so old I REALLY feel like i'm mixing the fics. If this sounds like a bunch of stuff to you please let me know. Just know its not Winters' Storm as I checked!!!