Spear of Heaven

The Emperor Estarion's long reign has finally brought peace, and even reconciliation, to the two warring halves of his Empire. But no Sun Lord can rest content in his triumph. Now Estarion and Vanyi, his Master of Mages, have the leisure to explore the rest of their wide world. Vanyi's Mage Guild has created a series of Gates that can take a traveler halfway around their planet in only a few steps.
Daruya, Estarion's willful granddaughter and heir to the throne, has wheedled permission from him to travel with Vanyi to the far side of the world - to a high mountain kingdom at the end of the chain of Gates. Daruya will be Estarion's ambassador, while her own daughter and heir remains behind.
But on the eve of their departure, the Gate in the Kingdom of Heaven is shattered by unimaginable forces, trapping its Keeper and sending a shockwave of power down the World Road to the heart of Estarion's city. Suddenly Vanyi's pleasant embassy becomes a dangerous overland journey to discover who can wield such power. And though Daruya is forbidden to go, she disobeys and hides among Vanyi's guardsmen.
It is as well that she does - for inside the Kingdom of Heaven, mere magery will meet its match. Vanyi will need the power of the Sun Lords, and the surprising alliance Daruya discovers, to counter the Breaker of Gates.