The Von Stein family had another race's territory nearby, near its border, the orcs. They weren't known for having a good temper but never did act against the vampires — probably due to the fear of getting retaliated.
But this time, they did. No one knew the reason behind the sudden attack from the orcs, not even the rank-four mages in charge of the Von Stein family, but they would not let them attack without striking back.
That was why Arthur and the other captains left the castle, looking to fight against the orcs or find out why the attack occurred. Duke Mikael did not go with them and only marked the spot on Arthur's map, leaving him alone to deal with all the other captains — who were not trying anything yet, but they would, eventually.