A king may also do this but can get stuck in a void and be found nowhere on the map (even with cheating you can’t find him). Lords from factions without any fiefs spawn randomly on the map. I got this bug and thought about the problem. This occurs very often in this old game and prevents players from winning the campaign. The king of this faction went missing and even his underlings do not know about his whereabouts.
You conquered everything but one faction remains still in the game. Here is my unorthodox trick to solve this problem! Solution This issue is old as the game and prevented many people from conquering officially the whole of Calradia. Even his own group remarks that he is no where around. The last king of the last remaining kingdom went missing. But there is no message about honor and victory. When every faction but one is defeated, all fiefs conquered, many people wait to win the game.