Children of the Changeling Series by Greg Keyes, J. Gregory Keyes

When her favorite cousin disappears, Hezhi, an exotic princess whose blood bears the seeds of the River God's magic, embarks on a dangerous quest to find the truth, a search that leads her to Perkar, a chieftain's son who has sworn to destroy the dreadedRiver God.

The River flowed from the mountains to the distant sea, and everywhere he touched, he ruled. And his blood ran thick in the veins of Nhol's Royal Family. But the god's power had a price: some members of the Family were never seen after puberty, and dark rumours abounded concerning their fate. When the magic woke in her, Princess Hezhi fled for her life into the lands beyond the River's reach... moreThe River flowed from the mountains to the distant sea, and everywhere he touched, he ruled. And his blood ran thick in the veins of Nhol's Royal Family. But the god's power had a price: some members of the Family were never seen after puberty, and dark rumours abounded concerning their fate. When the magic woke in her, Princess Hezhi fled for her life into the lands beyond the River's reach. With Perkar, a youth in search of honour, and loyal Tsem, her half-Giant bodyguard, she sought refuge among the barbarian Mang. But in those demon-infested hinterlands, until she learned to wield the powers of her birthright, her sanity and her soul would be at risk. And grisly danger pursued her - the River bent all his might and slumbrous cunning to the task of finding his wayward child. Only the Blackgod knew how to defeat the River. But the Blackgod was a creature of guile and limitless duplicity; to trust him might be the most perilous move she could make ... less