Never has Xanth faced so dire a threat. While the brutal Mundanes amass for attack on the northern border, Prince Dor takes charge of the Xanthian army, only to fall prey to a mysterious and total paralysis after the first battle. The fate of Xanth lies in the hands, or rather hooves, of the gifted night mare named Imbri. She has been charged with two vital messages: BEWARE THE HORSEMAN and BREAK THE CHAIN. Xanth seems doomed to destruction - unless Imbri can decode the messages and puncture the nightmare in time.

The Nextwave of barbarian warriors was invading Xanth from the north, ravaging and destroying as they advanced. But Mare Imbrium had her own problems. Ever since she had gained the half soul, the might mare had begun to mishandle her job of delivering bad dreams. Now the Night Stallion dismissed her, exiling her to the day world with a message for King Trent: "Beware the Horseman!" She had no idea of what that meant. But that was the way with prophetic warnings, nobody could understand them until it was too late. Then she met the Horseman. And she discovered that one who would ride a night mare was a master of bit and spur, and not a man to surrender her. For the might mare, it began to be all a horrible nightmare!

Summary:

War is 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 who 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.

"Imbrium," he tells her, "I am transferring you to day mare duty. A more horrendous mare will assume your night duties." The thought of her clients being placed in the power of one of the more brutal Night Mares pains Imbri deeply. But the Stallion knows what he is doing. Because she possesses part of a soul, he informs her, she is able to withstand the light of day, something the truly soulless Night Mares cannot do. Though her special night powers will be inoperable in the light, Imbri is still uniquely suited for a most important task: to deliver a message to King Trent at Castle Roogna. "Beware the Horseman," it says.

Imbri has no idea what it means. But that is the way with prophetic warnings- nobody can understand them until it's too late. Then she meets the Horseman, and she discovers that one 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!