So, there are some subjects that are mentioned as being taught at Hogwarts, that don't have professors mentioned as teaching them. I'd like a fic where Grindelwald escaped Nurmengard years ago and headed to Hogwarts, either to be near his former friend or to take revenge. He disguised himself as the Arithmancy teacher, because no one pays much attention to him, and has been teaching there ever since.

Dumbledore has no idea. However, during book 4, Grindelwald starts hearing rumors about Voldemort trying to come back, and doesn't want this to happen. He starts doing what he can to protect Harry, and, ends up showing up in the cemetery in time to stop Voldemort from returning. It's at this point, that Dumbledore begins to think that maybe his employee is a bit too powerful for just an arithmancy professor...