The Demon Court by Emma Hamm

He is temptation, but she will not be tempted.

As a child, Selene was left at the foot of the White Tower—given to the sorceresses who train those with magic. Her life has been spent honing her powers. But she can’t fully join their ranks until she fulfills a single, impossible task. Bring the Demon King, Lust, to his knees.
Lust has ruled his kingdom for centuries. He is the reason his people live without remorse and indulge in their senses. Life is mundane and boring…until he runs into a dark haired enchantress with eyes the color of the night sky.
With one whispered sentence, she changes everything.
“I’m the beginning of something new.”
Suddenly, the weight of centuries consumes him. He tries to use his powers of lust to sway her. He can bend the emotion, manipulate it, use that lust to control anyone he wants. But she’s a blank slate. He can’t feel anything inside her mind but ice and defiance.
All it took was one sentence to get into his castle, even less to capture his attention. And suddenly, the weakest of the sorceresses finds herself in a game of wits she has to control.
A game of deceiving a demon king while desperately trying not to fall in love with a creature who cannot feel anything but lust.