Night Mare

War was hovering at the gates of Xanth. The Nextwave of barbarian Mundane warriors is invading from the North, ravaging and destroying as they advance. And the future of Xanth is placed squarely in the hoofs of a horse, the Night Mare Imbri.
But Imbri has her own problems. In the land of Xanth, all bad dreams are caused by Night Mares, creative little beasts that live in the world of the hypnogourds and emerge only to carry frightening visions into the sleep of the inhabitants of Xanth. Imbri is a NIght Mare, but not a particularly successful one. Ever since she acquired half a soul during the adventure recorded in Ogre, Ogre. she has been too kind-hearted to really terrify anyone. Finally, she is summoned by the NIght Stallion, the horse who rules the World of Night.
Because she pocesses part of a soul, he informs her, she is able to withstand the light of day. Though her special night powers will be inoperable in the light, she is uniquely suited to deliver a message to King Trent at Castle Roogna. ""Beware the Horseman," it says.
Imbri has no idea what that means. But that is the way with prophetic warnings - nobody can understand them until it is too late. Then she meets teh Horseman, and she discovers that who would ride a Night Mare is a master of bit and spur and is not a man to surrender her. And that the power of this Horseman is just a taste of the nightmare that will come should his followers breach the defenses of Xanth.