OK, I've been looking for a specific claim fic and I am beyond frustrated that I can't find it! I just know it's probably one I should remember the name of and I can't. Plus search returns like 60 threads with the word "claim". I've given myself a headache going through all these stories! If someone can help me, I don't know what I can do to repay you but I'll be in your debt! (Maybe I can send you a good recipe or something?)
Anyway. Season 4 or Season 5. I think Buffy claims Spike first, but I'm not entirely sure about that. What I remember is a scene where Spike is in his crypt and he can't stop... cough... masturbating.

Buffy walks in and he is compelled to have sex with her so that one of them can reciprocate the claim (can't remember which one.) I do remember that Buffy tries several things to get Spike to, you know, *finish*, and finally she figures out he needs to bite her or she needs to bite him.
I *think* there's a scene at the end of the story where they are running around the graveyard in the rain? They both become stronger b/c of the claim. And I think Spike is able to go in the sun after the claim is complete.