This was one of the last Conan stories Howard wrote. Only four more would follow, but those four contain two of his greatest masterpieces, "Beyond the Black River" and "Red Nails." Howard's title was "The Servants of Bit-Yakin", but Farnsworth Wright changed the title to "Jewels of Gwahlur" when he published it in Weird Tales. That's the title it was known by until the Wndering Star/Del Rey editions, which restored the original title. However, there are some collections in print which are using the Weird Tales versions of Howard's stories, so you might find it under either title. Unlike some of Howard's work, there's no difference between his preferred version and the version that appeared in Weird Tales.
This isn't one of the better Conan stories. The plot requires swallowing a pretty large pill, namely that a treasure as valuable as the Teeth of Gwahlur (as they're called in the story) could remain unmolested in a lost jungle city for so long. Also, the heroine is way too hysterical. She's certainly no Belit.
The basic setup involves Conan working as a mercenary in the Black Kingdoms. He's there because he's heard rumors of a great treasure in a lost city, Alkmeenon, and is waiting around to find out the details. When an old enemy, the STygian Thutmekri, shows up and bribes some of the priests to take him to Alkmeenon, Conan is able to find out where it is. Knowing Thutmekri is working for the kingdom of Zembabwei and the whole thing is a set-up for an invasion, Conan leaves ahead of them.
Alkmeenon is hidden in a natural amphitheater surrounded by sheer cliffs. Not knowing the location of the secret entrance to the valley, Conan climbs the cliffs. Near the top he encoutners a small cave in which he finds a mummy holding a tube containing a parchment. Of course he takes it. This was one of the more powerful images in the story for me. Conan is hundreds of feet above the ground and comes face to face with a corpse.
I'm a sucker for lost city stories, particularly those that take place in jungles. The thing that makes this one unique is that so much of it is set in a series of caves and underground passages beneath the city. We know that Howard was inspired by a visit to Carlsbad Caverns when he wrote this one.
Another powerful scene, occurring about halfway through the story, is when Conan is attempting to sneak up on one of the people who have not followed but preceded him to the valley. Knowing the location of the secret entrance has its advantages. It's dusk, and Conan sees the white of his face contrasted against the darkness of the forest. When he approaches the man, Conan discovers its only the man's head he sees, tied to a branch by the hair. The valley is supposed to be deserted...
"TheServants of Bit-Yakin" (or "Jewels of Gwahlur" if you prefer) isn't the best Conan story, but it isn't the worst either. It's simply an adventure story, and a better than average one at that. While there are some problems with the characterization and some of the plot details, it still has its moments. In my opinion, it's worth reading.