 Under pressure from Napoleon both counties merged to become the Duchy of Nassau on August 30, 1806, under joint rule of Prince Frederick August of Nassau-Usingen and his younger cousin Prince Frederick William of Nassau-Weilburg.As Frederick August had no heirs, he agreed that Frederick William should become sole ruler after his death.However, Frederick William died from a fall on the stairs at Schloss Weilburg on 9 January 1816, and it was his son William who became duke of a unified Nassau.