Untamed (Primorian Crown #1) by Analise Song:
 
She was sent to kill him.
The dagger had other plans.

Sera is the Obsidian Hand’s deadliest assassin. There is only one rule in her Never miss—or the Hand will make sure you never breathe again. When her best friend misses his target, Sera makes a desperate Take an impossible contract to save his life. Her target is the fae prince whispered about in fear.

Caelan has spent two centuries guarding the border—an exile disguised as duty. Feared by the two realms for a magic called the Untamed, he had once lost control and turned the skies into permanent darkness. He had long since stopped expecting mercy…or surprises.

Until her.

Sera failed to kill him and instead accidentally bound them together with blood magic, turning hunter and target into allies. If he dies, she dies. He makes her his reluctant personal guard. Line between duty and desire becomes blurred.

They are supposed to find a way to break the bond but they find secrets instead.

Who sent Sera in the first place.
The permanent night sky is not what you think it is.
Some powers were never meant to be tamed.
And some bonds were never meant to be broken.

For readers who loved the world of A Court of Thorns and Roses but wanted a sharper heroine. The Untamed features a propulsive pacing of The Inheritance Games and the romantic tension of From Blood and Ash.