Bujold, Lois McMaster. Sharing knife #4

In a world where malices--remnants of ancient magic--can erupt with life-destroying power, only 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 Redwing Hickory rescued farmer girl Fawn Bluefield, neither expected to fall in love, join their lives in marriage, or defy both their kin to seek new solutions to the perilous split between their peoples