Make Her by Cassie Alexander

The final book in the Transformation Trilogy, an intense fantasy romance retelling of Beauty and the Beast:

Rhaim produced a key from somewhere upon his person. “The collar has a lock.” He tossed it out onto the table. “If I cannot solve the Deathless, I will become your loyal servant, until the end of my time.”
I was suddenly in danger of fainting. Rhaim had just as much as admitted he didn’t have an answer yet, and the price he would pay for failure was too great for me to bear.
I knew what it was like to be trapped in a cage.
I would never, ever, send anybody else—much less the man I loved—into one.

“Father, you cannot agree to his terms,” I told him. I rose up on my tiptoes, leaning over Helkin’s shoulder to shout, “I still hate you, beast!” willing Rhaim to change his mind.
Rhaim gave me a wicked smile, and spoke in the growl that bypassed my ears and went straight to my core. “As well you should, princess. And know, too, that if I am given you, I will never show you kindness again . . . and the things I will do to you will make the unicorns weep.”

Bend Her: A Dark Beauty and the Beast Fantasy Romance is the story of Rhaim the All-Beast, a cruel beast mage doomed to die at the hands of the woman he must protect at all costs, and Lisane, the sheltered Princess of Tears, who has been given into his care and who needs him to teach her magic so she can be free—no matter how much it might hurt her.