they respawn their armies at ANY castle,so if you let them have ONE castle,you will have to fight them again and again,until you lay siege to that castle,in one EPIC F**** AWESOME battle,because probably they all will be there. so keep conquering every castle you can,to weaken their garrisons,then hunt one lord by time. that's how i did it in my unified save. Don't know if it's the right choice anyway. sorry for engrish ._.