The Scepter's Return — Harry Turtledove

Two rival kings must unite to defeat a malevolent god in the “wise and humorous” conclusion to the Hugo Award�"winning author's Scepter of Mercy trilogy (Publishers Weekly).

When Avornis falls, the Banished One will reign supreme. Rival kings Lanius and Grus each covet solitary power, but realize that a kingdom divided cannot stand against the evil god who enslaves the minds of men through their nightmares. Once, Avornis was safe from his dark influence, protected by a powerful talisman supplied by the pantheon of deities who expunged the immortal called Milvago from their ranks. But that was centuries earlier, before the Scepter of Mercy was lost, and only through its recovery can Avornis survive. Now, Lanius has a daring -- most likely impossible -- plan, but it requires the cooperation of his fellow ruler, the warrior and usurper king Grus. If they fail to work together they will die together nevertheless, along with everything that is good and right in their world.
 
Hugo Award winner Harry Turtledove brings his Scepter of Mercy trilogy to a spectacular end with a story of courage, conflict, and selfless sacrifice. In The Scepter's Return, as in the previous books of the series, the acclaimed world-builder reimagines epic fantasy, leaving his own special imprint on the popular genre much as he has done with the alternate history for which he is so justifiably renowned.