Oathbreaker (Keepers of the Elusive Mysteries, #1)

Her birth sparked a war. She's vowed to end it.
Since she could hold a knife, eighteen-year-old Seren Moralis—bastard daughter of the enemy prince—has been fighting to prove her loyalty to the Primordian nobilis. After pledging to her uncle's House, she overhears his war plans to send a "weapon" to infiltrate the hidden holds of the Felinae. He must mean her. A surprise betrothal, however, has her running into the arms of her enemies under the guise of being the one thing she despises—a traitor.
Under the scrutiny of Llion Llewyn—a ruthless Felinae mercenary—and his young recruits, Seren bides her time spying, waiting until she can take her revenge on Prince Alban, the last royal, for the dishonour his uncle did to her mother. But as the lines between enemies, friends, and lovers blur, Seren discovers a shocking secret: her new "friends" have been hiding otherworldly abilities that make them living weapons.
When her uncle captures one of Llion's spies, Seren is forced to dance along the knife's edge between two oaths of loyalty. Even one misstep will have grave consequences. Yet all her sacrifices to gain her enemies' trust may be in vain for Llion harbours dark secrets of his own. And if the mercenary gets his way, Seren won't live long enough to step near the hidden prince, let alone break her oaths ever again.
Length: 481 pages