Endure (the Defy Trilogy, Book 3), 3

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Alexa and King Damian are engaged to be married, but the kingdom of Antion is besieged, their friend Rylan is a prisoner of the enemy, and Alexa has not told Damian that she is at the mercy of the evil Rafe, bound to obey one command of his choosing--but now Alexa must travel deep into enemy territory and confront an army of black sorcerers to rescue Rylan.