The Mirror Empire - Kameron Hurley

On the eve of a recurring catastrophic event known to extinguish nations and reshape continents, a troubled orphan evades death and slavery to uncover her own bloody past. As the dark star of the cataclysm rises, an illegitimate ruler is tasked with holding together a country fractured by civil war, a precocious young fighter is asked to betray his family and a half-Dhai general must choose between the eradication of her fathers people or loyalty to her alien Empress. Through tense alliances and devastating betrayal, the Dhai and their allies attempt to hold against a seemingly unstoppable force as enemy nations prepare for a coming together of worlds as old as the universe itself. In the end, one world will rise
and many will perish.