[1] In his late 50s, King Conan of Aquilonia engages in his final struggle with his arch-foe, the black magician Thoth-Amon of Stygia, servant of the evil god Set.

First, Conan must journey into Hyperborea and rescue his kidnapped son, Prince Conn, from an unholy alliance between Thoth-Amon and the witch queen, Louhi.

Next, Conan and Conn carry the struggle to their enemy's stronghold in Stygia itself at the head of an invading army, with the aid of a white druid named Diviatix.

Pursuing their defeated foe southward, they confront him again, first in the kingdom of Zembabwei and, at last, near the very edge of the world, where Conan and Thoth-Amon face each other in a final astral duel.

Critic Don D'Ammassa writes "Although this is billed as a novel it is actually a series of four novelettes set during Conan's tenure as king of Aquilonia.