Horizon — Lois McMaster Bujold

A Lakewalker entrusted with protecting the populace from malices -- terrifying remnants of ancient magic -- Dag Redwing Hickory never expected to fall in love with Fawn Bluefield, the farmer girl he rescued. When they joined in marriage, defying their kin, they bridged the perilous split between their peoples. Now Dag's extraordinary maker abilities have grown -- along with his fears about who and what he is becoming, and his frustration with the disdain in which Lakewalker soldier-sorcerers are expected to hold their farmer neighbors.

Fawn and Dag's world is changing, and the traditional Lake-walker practices cannot continue to hold every malice at bay. At the end of their long journey home, the pair must at last answer the question they've grappled with for so long: When the old traditions fail disastrously, can their untried new ways stand against their world's deadliest foe?