Complex sentence: From 1447 he was also Viscount of Narbonne and through his marriage to Eleonor, heiress of the Kingdom of Navarre, held the title of Prince of Navarre.
Split into simpler sentences: From 1447 he was also Viscount of Narbonne. Through his marriage to Eleonor, heiress of the Kingdom of Navarre, he also held the title of Prince of Navarre.