A Vow Made In Ash. The Ashbound Chronicles, #1

In the kingdom of Caldris, magic is woven into every promise. A spoken vow becomes an unbreakable bond, shaping lives, marriages, debts, and destinies. But Wren Ashdown carries a dangerous secret: she is Oathless, born immune to the very magic that holds the kingdom together. For six years she has hidden in plain sight as an archivist, determined to remain invisible after witnessing her mother's execution at the hands of the Inquisition.
When the ancient Wardstone that protects the realm begins to fail and a mysterious force known as the Hollow spreads across the land, Wren's carefully guarded life is shattered. Cassian Vale, the Crown's legendary Oathblade, is ordered to find an Oathless capable of saving the kingdom. Instead, he discovers Wren-and chooses to protect her. Bound together by necessity, Wren and Cassian embark on a perilous journey north in search of answers hidden in forgotten temples, ancient vows, and dangerous truths the Crown has concealed for decades.
As the Hollow grows stronger and long-buried secrets come to light, their uneasy alliance deepens into something far more complicated. Yet love may prove as dangerous as magic itself when one broken promise threatens an entire kingdom and the people sworn to save it must decide what they are willing to sacrifice. A Vow Written in Ash is a sweeping romantasy of forbidden magic, slow-burning romance, political intrigue, and the enduring power of promises.