Somebody Else's Magic — Marion Zimmer Bradley

Although it's Lythande's magic that's for hire, not her blade--at least, not the right-hand blade--you don't live long in the lands between Tschardain and Northwander if you don't have some skill with an edged weapon, and she has plenty. A swordswoman's mission, however, can sideslip without warning into a magician's ordeal. And may the gods help her who's caught in a web spun by somebody else's magic.