The Ascendance series

"When King Jaron returned to Carthya he brought with him a device that supposedly shows the way to a great treasure, but now everything is going wrong: Castor, the son of an old enemy, is conspiring against him and wants the throne for himself, his fianc?e, Imogen, seems to have turned against him, and the Prozarian Monarch has invaded his kingdom and taken Jaron's castle--but Jaron has a lot of experience at hiding and escaping, and he is not willing to leave Carthya or Imogen in the hands of his enemies"--OCLC.

Ascendant King Jaron believes that his kingdom, Carthya, is at peace, so he and his bethrothed, Imogen, are sailing home from a trade mission when their ship is attacked by Prozarians, and Jaron and several of his friends are taken prisoner; the Prozarian captain seems to believe he had something to do with his parents' deaths and they also know a great deal about Jaron's long-missing older brother, Darius, the rightful heir to Carthya--who may be alive after all.