Jhered of Kirea: The Rider in Black by Ron Prichard

Jhered of Kirea might have one day ruled the Insane Kingdom of Concubine; he rebelled instead and found a home in the cave city of Kirea. But when Concubine rolls through the Westernlands, all Jhered cares about is swept away. As he commits his soul to vengeance, he finds himself drawn into a world of ancient magics and dark sciences. Soon the soldiers of Concubine, who've never known defeat, begin to talk in fear of a Rider in Black.