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 . Duke Ernest Augustus I of Saxe-Weimar , Wilhelm 's second cousin , inherited Saxe-Eisenach ; he and his successors ruled Saxe-Weimar and Saxe-Eisenach in personal union until 1809 , when the duchies were formally merged into the Duchy of Saxe-Weimar-Eisenach .