Robert E. Howard 
The Hour of the Dragon 
Conan the Conqueror

The plot is a loosely based melange of motifs from previous Conan short stories, most notably 'The Scarlet Citadel’, with which it shares an almost identical storyline. It takes place when Conan is about forty-five, during his reign as King of Aquilonia, and follows a plot by a group of conspirators to depose him in favor of Valerius, heir to Conan’s predecessor Numedides, whom he had slain to gain the throne. To accomplish this they resort to necromancy, resurrecting Xaltotun, an ancient sorcerer from the pre-Hyborian empire of Acheron. With his aid the Aquilonian army is defeated by that of the rival kingdom of Nemedia and occupied. Conan, captured, is slated for execution until the sympathetic slave girl Zenobia risks her life to free him.
 
Conan’s quest to retrieve the Heart of Ahriman in order to defeat the wizard and regain his throne takes him through all the lands of Hyboria…