First published in Weird Tales all the way back in August 1929 (Volume 14, No. 2), this story is the first appearance of Kull of Atlantis, a character Robert E. Howard would write several short stories about and who would be part of the inspiration of his later character, Conan.

It's a major milestone in the sword and sorcery genre, and you can read it free:
https://en.wikisource.org/wiki/Weird_Tales/Volume_14/Issue_2/The_Shadow_Kingdom

This story is short but it does a lot. It establishes the character and background of Kull, some of his major allies, and one of his major motivations: to fight the serpent-men!