Love, Nemesis by A.K. Chandler

Survivors brought three warnings from the lands of the north.

First, they told of the dark, alien force that had broken the world.
More urgently, they warned of its human hosts, the Strike.
Third, they predicted a war for the soul of humanity.

Years later, those who survived the war won’t speak of it, and Ana is a hostage of that silence. Once a slave to the Strike, she’s found a new life in the Numbers. Using their own lifespans to fuel weapons against the darkness, the Numbers are heroes, but Ana’s past comes back to claim her when she meets Lethe Shepherd.

Lethe is one of the Riders of Saint East, a cult credited with ending the war. A prisoner of his own demons, he’s hunting the last Strike. Most want him buried with the past, but to Ana’s dismay, their histories bind them, and history is far from over.
People declare the war won, but rumors tell another story. Forced into a mission with unlikely comrades, she and Lethe soon find that settling their differences will mean the difference between life and death, for them, and the world as they know it.

The problem is, they’re enemies, and drawn together by hatred, it might just be love that tears them apart.