Complex sentence: The Nevilles and Percies burried their old differences for the Rising of the North, they wished to assert the claim of Catholic Mary I of Scotland whom they believed to be the legitimate monarch of England rather than Elizabeth Tudor.
Split into simpler sentences: The Nevilles and Percies buried their old differences for the Rising of the North. They wished to assert the claim of Catholic Mary I of Scotland, who they believed to be the legitimate monarch of England rather than Elizabeth Tudor.