Blood Claim

Valentine Greyfeather is a tank: a mage trained in destructive magic and usually the defensive force behind a partner trained in more delicate spells. Antisocial as he is, however, Valentine has gotten away with working by himself and answering to almost no one, until he's paired up with the big boss's adopted son, Kellan. Their rocky partnership is quickly put to the test when an underwater demon living in the nearby lake lays claim to Valentine.
The only thing that can save him is a stronger claim - like one made by his new partner.