A Fire in the Flesh by Jennifer L. Armentrout

The only thing that can save the realms now is the one thing more powerful than the Fates.
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\nAfter a startling betrayal ends with both Sera and the dangerously seductive ruler of the Shadowlands she has fallen madly in love with being held captive by the false King of the Gods, there is only one thing that can free Nyktos and prevent the forces of the Shadowlands from invading Dalos and igniting a War of Primals.
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\nConvincing Kolis won?t be easy, though ? not even with a lifetime of training. While his most favored Revenant is insistent that she is nothing more than a lie, Kolis?s erratic nature and twisted sense of honor leave her shaken to the core, and nothing could?ve prepared her for the cruelty of his Court or the shocking truths revealed. The revelations not only upend what she has understood about her duty and the very creation of the realms but also draw into question exactly what the true threat is. However, surviving Kolis is only one part of the battle. The Ascension is upon her, and Sera is out of time.
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\nBut Nyktos will do anything to keep Sera alive and give her the life she deserves. He?ll even risk the utter destruction of the realms, and that?s exactly what will happen if he doesn?t Ascend as the Primal of Life. Yet despite his desperate determination, their destinies may be out of their hands.
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\nBut there is that foreseen unexpected thread?the unpredictable, unknown, and unwritten. The only thing more powerful than the Fates?