Onyx Shadows

When Ellory's spirit is thrust into a noblewoman's body in a kingdom that executes magic users, she discovers a terrifying truth: she can wield forbidden magic.

Trapped in the royal court and betrothed to the prince, every moment threatens exposure. Her uncontrollable Gift keeps trying to reveal itself at the worst possible times, and the prince is growing suspicious.

...

For centuries, Maehdiorah has orchestrated the perfect plan to legalize magic. But when her chosen champion Elonrod discovers a girl touched by dark magic in the capital, Mae realizes her greatest mistake has returned to haunt her-and threaten everything.

Now Mae's centuries of planning hang in the balance, and Ellory must choose between the safety of hiding and the dangerous hope of freedom. But some mistakes can't be undone, and some prices are paid in blood.