A duke in Valyria is one of the highest level peer lords called magnates. Dukes are responsible for organizing the defense of their duchies, and appointing barons as vassals for that purpose. Militarily, dukes are considered to be colonels and the total mustered troops from their duchies are called regiments.
There are currently ten dukes in the Kingdom of Valyria. Dukes, by the right of their position as peers, are entitled to a seat in the House of High Lords. One duke takes one seat, but dukes have in Parliamentary Procedure. Dukes also convene to advise the King in the private council called Concilium Duxorum.
In formal address, a herald should announce a duke as “His Grace, the Most High, Potent and Noble Prince, the Duke of“, or “His Grace, the Most Noble Duke of” and directly as, “Your Grace.” Peers and superiors to dukes address them as “Duke N”, where N is their territory name or surname.