The Magic of Mortals by Cassia Meare

A woman from another time
A warrior from another place
Together they must save two worlds.
But first they must get magic …

The Wars of the Roses have only just ended in England, yet Lady Elinor Woodbrooke still dreams of knights and battles out of the window of her father’s castle.
Until the handsome Chevalier de Nemours appears. She can hardly suspect that he is a prince from another dimension searching for magical powers hidden on Earth by his people.
When he finds a star-shaped mark on her arm, he realizes that she is the mortal foreseen to play a crucial part in his quest.
Two worlds are at stake, and Nemours takes Elinor five centuries into the future to help him.
But now Elinor must learn magic to return to her time while dealing with Nemours’ powerful and cunning enemies.
The odds are against her, but one thing Elinor knows: A true English lady is never a coward.