"Red Nails" is the last of the stories featuring Conan the Cimmerian written by American author Robert E. Howard. A novella, it was originally serialized in Weird Tales magazine from July to October 1936, the months after Howard's suicide. It is set in the fictional Hyborian Age and concerns Conan entering a lost city whose degenerate inhabitants are entangled in a murderous blood feud. Due to its dark themes of decay and death, the story is considered a classic of Conan lore while also cited by Howard scholars as one of his best tales.

In the jungles far to the south of any known civilized or barbarian kingdoms, Valeria of the Red Brotherhood flees persecution after murdering a would-be rapist. She is followed into the wilderness by Conan, a fellow adventurer who wishes for an alliance with Valeria as her lover. Conan's stand-off with Valeria is interrupted by a dragon (actually a dinosaur, described with the characteristics of a Stegosaurus and Allosaurus) which mauls their horses. Conan and Valeria escape the dragon by climbing onto a rocky outcropping. Though they are trapped there without food or water, Conan recognizes some poisonous fruit growing nearby. Acting quickly, he coats the tip of a spear in poison and pierces the dragon's lower jaw with a well-aimed throw. Although blinded, the enraged beast pursues the two fugitives by their scent, but dies from the poison seconds before devouring Conan.

The couple emerge from their shelter and journey towards a mysterious walled city, which Conan sighted from the hill. Without grazing livestock or cultivated fields, the city appears deserted. Conan pries open a gate, long since rusted shut, and they enter a bizarre twilight world. The city, which is known as Xuchotl, is a massive structure completely enclosed by an emerald dome. A single great hallway runs across the entire city, without other streets or open courtyards. The structure consists entirely of four levels of rooms, chambers, and passageways. Xuchotl itself is carved from jade and other exotic materials.

The two separate and search the city's empty corridors. Valeria encounters a man named Techotl, who she joins in his feud between the two factions which are all that remain of the once large population. Soon, Techotl invites Conan and Valeria into the stronghold of his tribe, the Tecuhltli, where are welcomed by the king and queen, Olmec and Tascela.

Olmec recounts that the city was built centuries before its current inhabitants arrived. One day, a slave—Tolkemec—betrayed his master and guided the newly arrived invaders into the city while slaying the original inhabitants. The conquerors were led by two brothers, Tecuhltli and Xotalanc, who ruled peacefully over their city until Tecuhltli stole Xotalanc's bride. Meanwhile, Tolkemec betrayed both sides for his own reasons and was exiled to the catacombs. Nails driven into a pillar inside of Olmec's stronghold keeps count the number of slain rivals, and provides the title for this story.

Tascela develops an interest in Valeria, and has a slave try to drug her with a narcotic plant. Valeria manages to capture the slave and interrogates her into revealing her mistress' treachery, but the slave escapes into the catacombs. Valeria's pursuit is interrupted when Xotalanc's army breaches the stronghold. Eventually, all of Xotalanc's troops are exterminated while Conan, Valeria, Olmec, Tascela, and fifteen Tecuhltli warriors remain alive. When Conan begins an expedition towards Xotalanc's stronghold, Valeria is left behind while her wounds are treated.

While Conan is away, Olmec tries to rape Valeria, but he is thwarted by Tascela. She reveals herself as a sorceress and the stolen bride who originally started the feud. Vampire-like, Tascela plans to sacrifice Valeria to restore her own youth. Olmec has secretly ordered his guards to execute Conan, but Conan kills the two warriors and hurries back for Valeria. Returning to Tecuhltli, Conan finds a bruised Olmec in a trap inside Tascela's dungeon. After rescuing him, Olmec attempts to betray Conan and is killed.

Conan faces off against Tascela, who has Valeria chained on an altar. Caught in a steel trap, Conan watches helplessly as Tascela proceeds with her ritual. Suddenly she is interrupted by Tolkemec, returned from his exile and wielding an ancient sceptre shooting lightning bolts. Desperate for assistance against her nemesis, Tascela releases Conan, who manages to grab the scepter and kill Tolkemec. After freeing herself, Valeria impales Tascela with a dagger through her heart, declaring, "I had to do that much for my self-respect!" With the last inhabitants of Xuchotl dead, Conan and Valeria depart the empty city and make for the pirate coast.