The Keeper by ronsard (Katekyo Hitman Reborn; Yamamoto Takeshi/Gokudera Hayato; NC-17).
It is by far the best fic I've read (in any fandom) in what feels like forever. The quality of writing is so superb that you don't even really need to be familiar with the manga to appreciate this story (familiarity with the character backgrounds helps, but isn't required, because it's fairly easy to infer the important stuff). I am in total fangirl heaven. Damn, I've missed the place.
It's actually funny, but I have no real inclination to write in this fandom at all. I mean, there are things I could write, sure, but I don't really want to. I am just happy to have fallen in love with a character again, and Gokudera is fucking awesome. :">
Anyway! We now return to your regularly scheduled HP-centric programming.