Although the entirety of Robert E. Howard's Conan series is a seminal inspiration for Dungeons & Dragons, the 1936 story "Red Nails" is almost certainly one of its most direct inspirations. The story tells of Conan's discovery of an ancient walled city called Xuchotl, where he and his companion, Valeria of the Red Brotherhood, encounter the remnants of the decadent culture that once lived within its now-crumbling halls. Divided into two warring factions based on long-held grievances, Xuchotl proves to be a dangerous place for the pair, particularly Valeria, who arouses the nefarious interest of a woman named Tascela, whose role in the feud that wracks the city is central. Together, Conan and Valeria must get to the bottom of the mystery of Xuchotl and escape its many dangers.
"Red Nails" is the last Conan story Howard ever wrote. It was published posthumously and is widely regarded as one of the best of the series, if not the best. Howard himself was very fond of it himself, telling Clark Ashton Smith that it was "the grimmest, bloodiest and most merciless story of the series so far. Too much raw meat, maybe, but I merely portrayed what I honestly believe would be the reactions of certain types of people in the situations on which the plot of the story hung …" Besides its obvious pulp appeal, the story possesses some excellent meditations upon death, decay, and the decadence of civilization -- all classic Howardian themes, of course, but "Red Nails" takes them to a whole new level, in my opinion.
"Red Nails" has been very influential, serving up a terrific setting that's been imitated widely. D&D's own The Lost City clearly owes a great deal to this short story and David Cook acknowledged that it was also a prime inspiration for his own Dwellers of the Forbidden City, which is probably my favorite AD&D module of all. If you've never read it, I highly recommend it and not just for its historical value. "Red Nails" is a genuinely good tale in its own right and a fine introduction to Conan the Cimmerian.