The Assassin's Edge by Juliet E. McKenna
After a long winter spent in the Kellarin colony, the crafty and beautiful Livak is anxious to move on. Now an opportunity is on the horizon. The reclamation of a lost southern settlement is in the offing, but those involved, Livak included, must await the spring arrival of the first ship from the mainland -- an event that will never take place. Unbeknownst to all, the vital trading route to Tormalin is no longer secure. A dire new threat to the colony's survival has arisen. A final battle of strength, cunning and courage challenges Livak and her devoted swordsman-lover Ryshad, one that will force them to take up arms to confront a merciless, many-faceted evil.