Another duty that developed for the herald was that of being a “messenger of Peace and war”–a duty that developed during the reign of Edward III and must have grown out of or been added to their first duties of conducting tournaments or acting as masters-of-ceremony for their lords. From this time, heralds—at first rarely—began to be sent on military and diplomatic missions. They would carry letters and messengers from place to place as the official representative of their masters’. When Edward III announced that he would be founding a new order of knighthood, he sent his heralds to France, Scotland, Burgundy, Flanders, Brabant, Germany and Lombardy. During battle they would rouse the soldiers, place the separate feudal contingents in order of precedence and record the casualties of the battle sometimes with the heralds of both armies observing the battle and doing their duties together.

(Wagner, Anthony. Heralds and Ancestors. Great Britain: British Museum Pulications Ltd, 1978. p14.)
This is a depiction of a herald acting as a messenger. The Ambassador of Britain, here painted as a fifteenth century herald wearing a tabard of the royal arms of the time is presenting a letter to King Maurus. This is one of a fifteenth century series of paintings of the legend of St. Ursula.