The Starlight Blade by Nancy O’Toole

The Warrior Princess
Viola Verdis would do anything to protect her family. But what can she can do against The Mage King—an ancient enemy who hides in the shadows and uses others as pawns? With the royal family scattered for their own protection, it falls to Viola to protect her father, King Valient. But The Mage King’s latest weapon has plenty of tricks up her sleeve.
An Assassin in Disguise
Rue regrets joining The Mage King in his quest to sacrifice the royal family. And now, his magic burns in her chest, tying her to his will. By day, she disguises herself as a kitchen maid, masked by her cloak of many furs. By night, she plans King Valient’s demise, armed with the weapons of her late mother, Kelvia’s most notorious assassin. Only one person stands in her way, the king’s ever-present and infuriatingly beautiful daughter, Viola. Viola draws Rue in like no one has ever done before. But following her heart is impossible when she knows what she must do.
Spill royal blood, or die.