The Assassin of Grins and Secrets. The Grinning Assassin Trilogy, #1

Once the infamous assassin who killed with a smile, Serein now waits for death in the fighting pits of the Harpy's Chest. When the crown prince of Sarddon, Rameses al-Amirmoez, seeks her out with an offer of freedom if she serves as his guard, she must decide whether to work for those who ravaged her homeland or die. Rameses wants to believe that the war his father started was justified, but he has heard rumors and seen a cruelty in his father that causes him to question everything he's been told.
To find answers, he needs Serein. Uriah Stormheir, Captain of the Oyon's city guard, finds himself caught between his duty to the crown and his friendship with Rameses. His distrust for Rameses' new guard runs deep. As Uriah wrestles with guilt from his past, he must work alongside the dangerous assassin whose true motives are hidden underneath her scars and dark grins. In a land of hot sands and dying magic, Serein plays the game of secrets with Uriah and Rameses, knowing one wrong step could end her life.
As the shadows of her past catch up with her, she has to choose between pursuing vengeance and protecting a secret only she knows.*Trigger Warning* Please be aware that this book contains scenes of gore, violence, torture, physical abuse, sexual assault, mentions of rape, gaslighting. This book might not be suitable for younger readers.