The Blue Sword book by Robin McKinley

When Harry Crewe's father dies, she leaves her Homeland to travel east, to Istan, the last outpost of the Homelander empire, where her elder brother is stationed. Harry is drawn to the bleak landscape of the northeast frontier, so unlike the green hills of her Homeland. The desert she stares across was once a part of the great kingdom of Damar, before the Homelanders came from over the seas. Harry wishes she might cross the sands and climb the dark mountains where no Homelander has ever set foot, where the last of the old Damarians, the Free Hillfolk, still live. She hears stories that the Free Hillfolk possess strange powers -- that they work magic -- that it is because of this that they remain free of the Homelander sway. When the king of the Free Hillfolk comes to Istan to ask that the Homelanders and the Hillfolk set their enmity aside to fight a common foe, the Homelanders are reluctant to trust his word, and even more reluctant to believe his tales of the Northerners: that they are demonkind, not human. Harry's destiny lies in the far mountains that she once wished to climb, and she will ride to the battle with the North in the Hill-king's army, bearing the Blue Sword, Gonturan, the chiefest treasure of the Hill-king's house and the subject of many legends of magic and mystery. Published By Harper Collins on 1982-10-01