There are not a great many underwater undead, and there are as many water elementals that can search the riverbed for skeletons, so Ksiaze does not, actually, have a river ambush planned; the undead cutting off their line of retreat were mindless fools or nameless allies, not his disciplined legions, for he intended to make Ardeal the center of his return to power, not simply a scene of slaughter.
- But there was chaos in Ardis. The night of blood in Ardis was not a long night, with so much of the garrison south in Kavapesta; when it ended the duchess still fanning herself in her chambers, glazed eyes straight ahead as she insists that there's no danger, Razmir promised he'd fix that, a very kind gentleman, while her daughters and their husbands bicker over the question of authority. The countess's knights were dead or gone, the Razmiran vicar, control of the countyside cut, officials in charge of tax administration all dead, the town guard slain at their posts or fled. In a sack, all crimes become legal, property up for whoever can hold it; no one can stop fires if they burn out of control, or shield the innocent from murder and assault without greater strength of their own.
Under these circumstances, it's natural to turn to those people who have actual experience with dangerous situations, and who might have survived the night of blood when the vampires attacked.
That is to say, retired adventurers.