The Bastard King — Harry Turtledove

Two rivals must unite to prevent a disgraced malevolent god from attaining a relic of extraordinary power in the first book of alternate history master Harry Turtledove's magnificent fantasy trilogy

More than four centuries ago, the Scepter of Mercy was lost to the king of Avornis, and each subsequent liege has promised -- and failed -- to return the powerful talisman to its rightful home. Now, young Lanius, the only surviving son of King Mergus, rules, though he is considered illegitimate and must abide by the decisions of regents. Still, the legacy of the missing scepter ultimately belongs to him. But it is also coveted by the Banished One, an immortal exiled by the other gods, who invades the world of men through their dreams. Lanius, with no talent or heart for battle, must keep those in the sway of the malevolent deity from Avornis's borders. To this end, Lanius requires the help of Grus, a fearless and respected captain of the king's navy. But Grus has a far loftier destiny than his common birth would suggest -- and the bastard king's brave, accomplished ally might well turn out to be his most dangerous adversary.
 
Originally published under the pen name Dan Chernenko, The Bastard King is a magnificent foray into epic fantasy by the incomparable Harry Turtledove, the prolific and multi-award-winning master of alternate history science fiction. A tale of courage and destiny, it is alive with action, imagination, and humanity, and populated by richly complex, imperfect heroes and a villain as truly fiendish as any that has ever graced the fantasy genre.