My Wife, The Crimson Marshal

Crimson Marshal Seraphina Shaw hides identity to marry into House Langdon, enduring neglect. After annulment, she dons armor, defeats twenty thousand Tartars with twelve hundred cavalry, and is appointed Grand Marshal of the North. She investigates a case to uncover the traitor, implements military reforms for the Northern March, and joins hands with her deputy, Ethan Peyton, who protected her.