The Heir Comes Forth by R. S. Gullett 

The king has been murdered, a usurper seizes the throne, and the kingdom faces utter destruction unless the true heir can be found. Six individuals from different walks of life are drawn into the quest to find the lost heir and help him in his claim to the throne. A young man seeking adventure, a repentant assassin seeking to find his lost sister, a princess trying to discover her father’s killer, a usurper seeking revenge for his family’s murder, a young man yearning for approval from his father and seeking his place in the kingdom, and an exiled warrior trying to restore his honor and rescue his beloved.
Finally finding the heir, the kingdom is torn apart as people choose whether to support the usurper or the true heir. Each individual is confronted with a choice also: to seek their own or take a chance on faith in something greater.