The Golden Hills of Westria by Diana L. Paxson
Prince Phoenix has always been a bit restless, never quite measuring up to the expectations of his father King Julian. Sombra, Phoenix's childhood friend, believes in him, but she has her own duties as a student of the College of the Wise. Their relationship surpasses friendship, but their responsibilities take precedence over their feelings. Phoenix is abducted in a raid and sold to slavers. Enduring pain and humiliation, Phoenix must find a way to survive, even if that means becoming a danger to the people that he loves. While King Julian searches for his lost son, a growing army of fanatics led by the charismatic Mother Mahalial is conquering land after land, and its final destination is the golden land of Westria. The only person who can sense Phoenix's plight is Sombra, and her attempt to rescue him may decide the fate of the kingdom.