The title for king, but more accurately chieftain, among the Britons.
An arglwyd of the period generally controlled a small parcel of land, perhaps no more than twenty-five miles in diameter. These might have a small village of no more than a hundred persons every five to ten miles along the public roads.
A king was expected first to protect all the villages under its rule from raiders, provide food and supplies in times of famine and disaster. Among his men, a king was supposed to lead in the hunt, be a generous host during feasts he hosted. He was expected to successfully lead his warriors in battle and defend his own cattle against raids.
The arglwyd was believed to have a mystical connection to the land, and part of the inaugural ceremony involved his marriage to his kingdom. It was believed that a new king married a beautiful young woman, a perfect representation of the land. As he became less virile she aged, and upon his death she would revert into an old hag.
It was also believed that the woman would divorce him if he did anything that was considered "unkingly", for instance turning his back on the enemy during battle, or losing any element of his person, e.g. a finger.
The rise of arglwyd's heralded the first unified resistance against the waves of Germanic refugees hoping to find land and overwhelm the natives.