Complex sentence: Madelyn then finds a book about the ancient staff of O'Flannery, the magician who brought over the castle, which is now the school, from Ireland, that controls the Griffin so they go looking for it.
Split into simpler sentences: Madelyn then finds a book about the ancient staff of O'Flannery, the magician who brought over the castle from Ireland, which is now the school. The staff controls the Griffin so they go looking for it.