Saxe-Eisenach changed again in 1672. Frederick William III of Saxe-Altenburg died and his lands were split. The line of Johann Georg I ruled Saxe-Eisenach for 69 years, until Duke Wilhelm Heinrich died heirless in 1741. Ernst August I of Saxe-Weimar, Wilhelm's second cousin, inherited Saxe-Eisenach; he and his successors ruled Saxe-Weimar and Saxe-Eisenach in a personal union until 1809, when the duchies were formally merged into the Duchy of Saxe-Weimar-Eisenach.