Traitor's Game by Jennifer,A Nielsen
Nothing is as it seems in the kingdom of Antora. Kestra Dallisor has spent three years in exile when a group of rebels snatch her as she reluctantly travels home. The kidnappers want her to retrieve the lost Olden Blade, the only object that can destroy the immortal king, but Kestra is not an obedient captive. Simon, one of her kidnappers, will have his hands full as Kestra tries to foil their plot. As motives shift and secrets emerge, both will have to decide what-and who-it is they're fighting for.