Title: Aurora Borealis
Pairing: Spike/Anya
Must have: Strange lights in the sky, Spike and Anya going 'undercover' in LA's demonic underground, and (if she's not a vengence demon at the time of the fic) a spell or potion to make Anya temporarily demonic.
Bonus: If you include some 'classic' spy stuff like the saying 'If I told you that I'd have to kill you.'
Summary: Spike and Anya immerse themselves in the underground scene to try and stop a prophecy from coming true.