Original: In 1668 he died, just before of the birth of his fifth son, Wilhelm August, who became the new Duke of Saxe-Eisenach from his birth, under the guardianship of his uncle Johann Georg.
Simplified: In 1668 he died, just before of the birth of his fifth child, Wilhelm August, who became the new Duke of Saxe-Eisenach from his birth, with Johann Georg as regent.