Magic Steps - Пирс Тамора

Sandry finds a dance-mage boy in Summersea, the twelve-year-old Pasco Acalon, the son and grandson of two police families (known as «harriers» in Summersea). When a rich trading family falls prey to a serial killer, she and Pasco must work together to stop the killer mages who have a deadly weapon — unmagic, which is the absence of magic and life.