Vasilisa

A pirate! HP fic starring the Dread Pirate Sirius Black, a swashbuckling Kingsley Shacklebolt, a reclusive Captain Tom Riddle and a strange ship that goes out of its way to be pirated. A mysterious book falls into the right hands, and three lives collide when Harry, Ron and Hermione find themselves on the S.S. Griffin.

It is one thing to go to the immediate past, but certain questions arise when one goes far enough. Can time be changed, or is the presence of the thing sent back just a recursive proof of the present? If things haven't reached their worst, do they need to be prevented? After her parents are killed and Harry disappears, Hermione loses enough to break the greatest rule of all.