Oathblood — Mercedes Lackey

TARMA
She was one of the Shin'a'in sword-sworn--the most elite of all warriors, and she had taken blood vengeance upon the brigands who massacred her people.

KETHRY
She fled her oppressive arranged marriage and became a powerful sorceress of the White Winds School, only to be bonded to a uniquely magical sword, Lich drew her to women in need

Bound together by sword-spell and the will of the Goddess, Tarma and Kethry swore a blood oath to carry on their mutual fight against evil and injustice.

Now they have pledged to train others to fight for their cause, starting a new school for young, would-be warriors and fledgling White Winds mages. But training young fighters and wizards turns out to be far more complicated and perilous than they expected--and when two of their prize students are kidnapped, Tarma and Kethry must draw upon all of their combined skills to answer the call of their destiny in ways they never imagined!