Complex sentence: The couple had many grandchildren who became emperors and kings of various countries, and the Kings of Spain and Emperors of Austria are obviously descended from their union, with their major dynastic heir being King Juan Carlos I of Spain, who reigns in their native territory.
Split into simpler sentences: The couple had many grandchildren who became emperors and kings of various countries, and the Kings of Spain and Emperors of Austria descend from their union. Their major dynastic heir is King Juan Carlos I of Spain, who reigns in their native territory.