From audible.com:

In a world where malices, remnants of ancient magic, can erupt with life-destroying power, only the soldier-sorcerer Lakewalkers have mastered the ability to kill them. But Lakewalkers keep their uncanny secrets - and themselves - from the farmers they protect. So when patroller Dag rescued farm girl Fawn, neither expected to fall in love, marry, and defy both their kin to seek new solutions to the split between their peoples.

As Dag's powers have grown, so has his frustration with the Lakewalkers' rigid mores. Fawn and Dag see that their world is changing and that the traditional Lakewalker practices cannot hold the malices at bay forever. Yet for all the customs that Dag and Fawn have challenged, they will soon be confronted with a crisis beyond imagining. When the old ways fail, can their untried new ways stand against their world's deadliest foe?