Konstantin is a vampire on assignment. He's one of the elite Nightwardens assigned—make that conscripted—to guard witches. He and the rest of his brethren serve one purpose. Bodyguards to the High Sorceresses. He shouldn't be plagued by blood lust. He shouldn't be yielding to emotions.
Monika is the daughter of a High Sorceress. To say she finds Konstantin attractive is to put it mildly, despite his brooding, dark, stoic front. She buys him an ancient sword, not knowing the price involved.
Until the one collecting on the price comes calling.