Hair of the Mongoose Fangs of the Wolf

AU: Sirius Black did something terrible in his fifth year at Hogwarts, and it all had to do with Severus Snape. He made sure Snape met transformed Moony that night in the shack, but it did not end the way he expected. Now, having been bitten by Nagini, Snape realises that he's undergone some changes... again. He also bit the witch trying to save his life. Now what? [HG/SS]