Crimson Heart by Heather McCollum

Elena has spent her entire life hiding her family name while walking the thin line between lady and servant in a Protestant household. She has kept the secret of her lineage for years, but with a new, fiercely Catholic queen on the throne, Elena's Protestant caretaker is being targeted. And when Bloody Mary discovers Elena's true identity, she must flee to Scotland to save her life.

Highland warrior, Searc Munro, has secrets of his own. In a family of healers, his dark, killing magic must never be known. When his father's life is threatened, Searc unleashes his lethal power to save him. With his clan's suspicious glances and whispers plaguing him, he sets off alone, only to find another use for his magic in saving a beautiful lass being attacked in the forest.

Teaming up as exiles, Elena and Searc head toward Edinburg to find refuge. Yet, with a series of ritual killings haunting the city and a traitor attempting to assassinate Scotland's regent, suspicion turns to Searc. As word of another British heir reaches the country, the two must learn to trust each other if they want to live.

The Highland Hearts series can be read out of order, but is best enjoyed in order.
Reading Order:
Book #1 Captured Heart
Book #2 Tangled Hearts
Book #3 Untamed Hearts
Book #4 Crimson Heart
Book #5 Highland Heart