Book Once Upon A Demon's Heart — K. M. Moronova

Haunted by her role in a massacre that reignited an ancient war, demigod knight Alira is cursed to relive her kingdom's destruction - dying again and again at the hands of the Blood Knight, a ruthless demon commander. But when the demons propose a peace marriage to end the bloodshed, Alira volunteers herself, desperate to break the cycle and save her people.
Betrothed to her enemy - who is also the lost demon boy she once tried to save - Alira must navigate a dangerous path between vengeance and forgiveness. Kalel Lornhelm, haunted by his own dark past and the monstrous hunger within him, is torn between hatred and a growing, forbidden attraction.
As assassins close in, gods interfere, and the threat of betrayal hangs over their union, Alira and Kalel are forced to confront their intertwined destinies and the curse that binds them.