Desa and Kalia by R.S. Penney

The Void had been defeated. It was supposed to be over.

Ten years ago, Desa Nin Leean put away her guns and settled into a quiet life with her partner, Kalia. But her world has gotten a lot more complicated ever since her people discovered that they weren't alone in the universe. Ambassadors and diplomats and Justice Keepers, always coming and going.

Now, an assassin has come to her city, wielding the ancient power of the Void, a religious fanatic who turns ordinary people into zealots. These foreigners with their fancy technology aren't prepared for a threat like this. Desa has no choice but to confront the assassin and discover the truth about his agenda.

But in doing so, she will come face to face with an enemy far more deadly than any fanatic: the guilt, anger and grief that she has suppressed for so long.