Complex sentence: Magnus'own daughter was in a marriage with a Norwegian nobleman, and the one descendant wed to the Earl of Orkney, through which lineage Magnus' descendant James VI of Scotland became King of England.
Split into simpler sentences: Magnus'own daughter was in a marriage with a Norwegian nobleman, and the one descendant wed to the Earl of Orkney. Through which lineage, Magnus' descendant James VI of Scotland became King of England.