Aria of the Oathkeeper

Yuki is taken by a mysterious kidnapper and Zero takes it upon himself to rescue her. Going through a portal to an unknown land, Zero finds himself at the entrance to a castle lorded over by a vampire named Carmilla. Though he could care less about her, he soon realizes that rescuing Yuki will put him right in the vampire's crosshairs. / Part of the "Blood Moon Nocturne" series