Mercedes Lackey

Acting as the "son" her father never had, Gwenhwyfar, when called upon to serve another purpose by the Ladies of the Well, bows to circumstances to become Arthur's queen--only to find herself facing temptation and treachery, intrigue and betrayal, but also love and redemption.

"A teenage girl leaves home to join the legendary Hunters, who protect the people from the terrifying monsters that have overrun their world"--