Titus Pwy was the only one of the fifteen students that survived an accident while making a philosophers stone in the late 1800, He absorbed part of the stones power and therefore ages only at about a tenth of the speed. From 1910 – 1939 he taught at Hogwarts (Dumbledore, Flitwick and McGonagal). In the summer bevor Harry’s fifth year he returns and helps Harry,Ginny, Ron and Hermione.
In the process of doing so he introduces them to the Old Belivers that separated them self from the rest of the magical world between 300 and 400 and emigrated in 1500 to North America.
DrT writes:
Professor Pwy, Albus Dumbledore’s mentor, returns to train Harry and his friends. While the Quartet trains, they also discover each other. When the Dementors ally themselves with Voldemort, Pwy brings in the separatist descendants of Druids and other Old Believers.
The story covers 5th through 7tn year and can be read at The Dark Arts.
I also made a PDF of the story.