I think this is from "Forgive Me" by Herself, part of the Bittersweets Series. Buffy doesn't stumble on it she takes it away from Dawn after she had retrieved it from Spike's destroyed crypt. It's a angsty story, to say the least.
Buggerthis.net crashed a while back, so I can't find a decent current link, except from All About Spike :
http://www.allaboutspike.com/fic.html?id=647Maybe the Wayback Machine or someone else????
Peyton, I used your word search method - "angelus". I knew it was Herself, but I couldn't recall which one.