Xanthe is the oldest vrakhir in Brencis's court, older even then the knyaz himself. While the others rely mainly on their claws and fangs, Xanthe prefers to meddle with her foes's minds. They say she can drive you mad in an instant… Or even make blood come bubbling out of your ears.
In Brencis's grand plan, she's responsible for turning soldiers into blood guards. How exactly she does that remains a mystery, but it involves a dark ritual that cannot be reversed.