The Fallen King’s Penitent Soldier by Megan Derr

The youngest son of a despised king, thrust upon a throne he never wanted, Desmond nevertheless tried to be a good monarch to Benta. Despite his efforts, the rebels discontent with him and the alliance with the Harken Empire have taken control, and Desmond’s private guards are either dead or turned traitor. On the verge of being executed, Desmond gets an unexpected rescue from Harken, in the form of the fearsome Penance Gate mercenaries and the beautiful, brutal man who leads them.
Beneath the shining edifice of every empire is a foundation of violence and pain, and Chass has always done his duty to Harken by being the bastard who endures and metes out both. He is used to being hated, and does not deny he deserves it. But in the aftermath of rescuing the enchanting, compelling Desmond, he wishes more than ever that he could be the noble hero just once, instead of the monster good only for spreading terror.