Ravenheart

In mid-1400s Scotland, the magical beings known as Slig Maith have had enough. Due to human waste and destruction, a war now brews, just as young Bernadette's life is shattered. She tries to save her mother from a witch's death but fails. Now full of pain and fear, Bernie makes a bargain with the Queen of the Slig Maith.
In exchange for magical abilities, Bernie will help the Queen win her war, but Bernie does not understand the full ramifications of her actions. She can only think of avenging her mother's death. Once the decision is made, Bernie's education begins. She is taught magic and martial arts by a Taoist immortal while a house elf teaches her reading, writing, and all the manners of a high class lady.
Over time, Bernie becomes more Sligh Maith than human. Despite doubts about her newly acquired skills, she is forced to seek revenge before she is ready. Bernie now owes the Queen everything. All she longs for is a normal life with a husband and family. She has no choice; she must follow through with her bargain and become a mercenary for the Queen or face dire consequences.