Conan has been leading a band of kozaks for some time, and it's causing no end of problems for the local Turanian governor Agha. Noting Conan's predilection towards Damsels in Distress, he decides to use his most beautiful slave, Octavia, as bait to lure Conan into a trap. Octavia, looking to escape her enslavement, warns Conan of the attack, and the two flee to a deserted island in the middle of a lake. Unfortunately, this was the governor's plan all along, and with his best men he follows them.
On the island, Conan and Octavia are shocked to find a city that wasn't there the day before, full of hanging tapestries, golden banquet halls, and beautiful sleeping people. Heading for the center of the city, they find its keeper, Khostral Khel, a millennia-old Humanoid Abomination who has just awoken from his long slumber, and who has now brought the city back to its former glory.
Trapped and with nowhere to run, Conan and Octavia must contend with a giant serpent, Agha's forces,and the horror of the devil in iron.
- Action Prologue: The fisherman breaking into the temple.
- As You Know: Ghaznavi, a Turanian counselor, tells Jehungir, a Turanian noble, of the island on which most of the action takes place, and how they lost a detachment of troops near it to Conan's kozaks. Lampshaded when Jehungir wryly replies "I am not likely to forget that."
- The Bait: Octavia, intended to so enrapture Conan that he'll come alone to an island on the rumor she'd escaped and awaits him there.
- Barbarian Tribe: The kozaks, made up of criminals, deserters, and other undesirables, who've formed their own society. Conan is leading them.
- Bedsheet Ladder: How Octavia escapes her captivity for true. Though again, with torn tapestries instead of proper bedsheets.
- Blue Blood: Octavia was a Nemedian noble before she was Made a Slave.
- Cold-Blooded Torture: Jehungir has a Shemite torturer, and just the threat of being turned over to him for an evening is enough to make Octavia's resistance to her part in the plan vanish. Octavia is then handed to Jelal Khan, "a nobleman whose name was a byword for degeneracy." We're not told what either of them did to or planned for the girl, but it's enough to prompt her to escape.
- Curiosity Is a Crapshoot: The fisherman rouses the evil by his curiosity.
- Damsel in Distress: Octavia.
- Darkness Equals Death: Inverted. Khosatral's necromancy only fully revives the population of Dagon at night, as they've been dead too long to be fully restored to true life. By day, they fall into a deathlike slumber.
- Decadent Court: What Octavia fled.
- Faux Death: Conan finds people rousing from it.
- Femme Fatale: Octavia was ordered into it.
- God-Emperor: Khosatral Khel, back when he ruled Dagon. Emphasis on god.
- Humanoid Abomination: Khosatral Khel, as Conan overhears him explaining to Octavia, is an abominable being spawned in the primordial chaos outside reality. The form he reverts to upon death is so ghastly that it causes Conan to flee in fear. And the name of the country he ruled? Dagon. His description is even the page-header quote for this very trope.
- Human Sacrifice: Often made to Khosatral Khel in Dagon's glory days.
- Intro-Only Point of View: The fisherman dies and so exits the story.
- Living Statue: Khosatral Khel is effectively one, indwelling in an iron body because no mortal flesh could properly serve as a vessel for his mighty essence.
- Lotus-Eater Machine: Conan at first assumes the city is this, due to the drugged, sleepy behaviour of the inhabitants. The townspeople actually act that way because they have been brought back after centuries of death, and Khosatral Khel expends immense amounts of power keeping their mental faculties working.
- Made a Slave: Octavia's Backstory.
- Made of Iron: The villain, literally, making him nearly invulnerable to everything except the magic dagger that had sealed him.
- Mercy Kill: Octavia begs for this when she and Conan are cornered by the apparently invulnerable Khosatral Khel (apparently Khosatral has threatened her with a Fate Worse than Death). Conan instead tells her that he will hold off Khosatral for as long as possible to give her a chance to escape the island.
- Necromancer: One of Khosatral Khel's many hats.
- Neutral Female: Octavia doesn't even try to help out while Conan is fighting the giant snake.
- Oh, Crap!: When Khosatral sees that Conan has the magic dagger, he seriously reconsiders his attempt to murder our favorite Cimmerian.
- Rescue Sex: Conan is surprised that Octavia rejects this and he has to seduce her.
- Sealed Evil in a Can: Khosatral Khel, until awakened by a fisherman stealing the dagger that was keeping him inert.
- Snakes Are Sinister: A forty-foot snake is one of the creatures doing Khosatral Khel's bidding.
- Temple of Doom: Where the trouble was unleashed.
- This Was His True Form: When Khosatral dies, he reverts to something so ghastly that it sends Conan himself fleeing in fear.
- Thunderbolt Iron: Enabled a slave revolt, providing a power capable of neutralizing Khosatral Khel's influence and wounding his otherwise-invulnerable body.
- Wait Here: Conan tries this on Octavia, warning her to stay outside a room with a giant snake in it while tries to sneak past it and get the magic knife. She doesn't listen, Hilarity Ensues.