Book Seven Deadly Thorns — Amber Hamilton

Viola Sinclair has hidden her shadow magic her whole life. Discovery means certain death, for in the Kingdom of Aragoa dark magic is forbidden. When the Queen finds out Viola's secret, she sends her royal assassin to kill her. He is given a cursed tattoo: seven thorns on his arm for the seven days he has to complete his deadly task. If he fails, he will be the one to die.
The assassin is Roze Roquelart, entitled prince, arrogant fellow student at Vandenberghe Academy and Viola's mortal enemy. The chance to strike Viola down is at Roze's fingertips. But he needs something from Viola. And he's willing to spare her life to get it.
Forced to work together, Viola and Roze uncover terrible secrets, sinister threats . . . and an irresistible attraction. Can they learn to trust each other in time to save themselves - and bring the light back to Aragoa?
Her worst enemy. His only chance.