The Lone and Level Sands

In an alternate ancient Mesopotamia 4000 years ago, queen Ninsun makes a deal with a god to have its child, and then gives birth to twins—Tarlos, the telekinetic heir to the throne, and Krastos, a demigod. On the twins' twelfth birthday, Ninsun is killed by a monster named Bawa.
Years later, the twins kill a roc and manticore as a trial of adulthood. At the subsequent feast, Krastos is framed for a crime and is imprisoned and sentenced to death. In order to clear his tarnished honor, Tarlos breaks Krastos out and the two hunt down the monster that killed their mother. They are successful in killing Bawa, but Krastos is killed.
Tarlos learns that his father orchestrated Krastos being arrested, and that Krastos's father was a different god than what everyone thought. It wasn't Moleg, the god of strength, but Ablis—the Discarded One. After Tarlos's coronation, Ablis comes to him and explains that Krastos was meant to be an only child and was to inherit the throne, giving Ablis control over the kingdom and ultimately the world. Now that Krastos is dead and Tarlos is king, the legacy of his kingdom will eventually fade into nothing. Ablis almost kills Tarlos, but decides to let him keep living with his fear of death, which he deems to be worse than actually dying.
Tarlos decides that he can prove Ablis wrong and also get rid of him by finding the secret to immortality. He journeys across the desert to find the gateway to the dead country, where he meets people from alternate versions of his own world. A man takes him down the river Styx where the dead country ends and the Ageless country begins.
Now in the Ageless country, Tarlos meets an immortal man and learns about the elder gods—beings who created Tarlos's gods. It is an elder god that Tarlos must ask for immortality, and Tarlos does just this. He again makes his way over a vast distance to find a special book that allows him to speak with an elder god. The elder god gives Tarlos the means to fight Ablis, then sends him home.
Back in Kesh, Tarlos discovers that Ablis has taken his throne and his likeness. He attempts to fight the god, but Ablis kills him, and Tarlos ends up back in the dead country. There, Tarlos learns that there are more important things in life than trying not to die, and he escapes the dead country to face Ablis one more time.
Now facing his own inevitable death head-on, Tarlos manages to banish Ablis into the Continuum, in the space between spaces. The elder god speaks to him again, offering Tarlos immortality, but Tarlos turns it down, choosing instead to live his best life and eventually die surrounded by loved ones.