What happens to Harry as her group retreat from the Northern army?

Harry is stopped by the rider on the white horse. The horse has the sharp, curved teeth of a flesh eater, and when the rider clashes swords with Harry, she feels the power of the him as it nearly knocks her from the saddle, and her right arm goes numb. Sungold rears and succeeds in wounding the rider. The white horse goes mad, and as she looks into the face of the rider, she hears him laugh and she realizes the creature is of demonkind.