Capturing a lord is very difficult, as they will usually escape when defeated. This can be achieved either through bribery (with the possibility of using persuasion to affect the price), using your good relation with the lord to force them to accept, or by capturing them. To complete this quest, you must convince the named lords from both warring factions to accept a peace treaty. You will be asked to stop the war between the Guild Master's faction and another faction within 30 days. The Guild Master's faction has to be at war for this quest to be available.
This is a quest given by the Guild Master in towns.