"Yuki-onna" by Stendec
Energy can neither be created nor destroyed. It can only change forms. In any process, the total energy of the universe remains the same. That is science's First Law of Thermodynamics. And it's wrong.
Set in an alternate universe that diverges from the Harry Potter novels at the end of Harry's second year. The Weasleys, wanting to let Ginny recover in a familiar environment, postpone their trip to Egypt. Sirius Black never sees a picture of Wormtail in the paper, and doesn't escape Azkaban… granting the crew a peaceful, uneventful third year at Hogwarts with a decent teacher. But what happens in Harry's fourth year, when all those events that should have happened come spewing forth, with that bloody tournament as well?