Reign of the Talon by Sophie Kim

How do you kill a prophecy?

The Prophecy has arrived…and with her, war.
A prisoner caged within the walls of her own mind, the once-fierce assassin known as Shin Lina can do nothing but watch as a tide of blood and chaos sweeps across the Three Kingdoms. After all, Lina is the one who unleashed the malevolent Prophecy upon her world.
So many secrets. So many lies. And it’s all her fault.
Yet Haneul Rui—the immortal Dokkaebi Emperor who stole her heart—refuses to surrender. He alone leads the armies against the dangerous, serpentine Imugi. He alone wields his scorching fire atop blood-soaked battlefields, and he alone rallies his soldiers against a terrifying future of death and destruction.
Now the red thread of fate ties them all together in love and hatred—Rui, Lina, and the merciless Prophecy herself. And the more Lina fights against her destiny, the tighter the thread becomes.
For they are bound in both life and death; to wound one is to wound them all. To kill the Child of Venom is to kill them all.
And soon, not even the wrathful gods themselves can stop their inevitable ruin.