When I decided to start working on my campaign world of Toldara I knew I had to include them. The main design goal of Toldara was to include stuff inspired by things I enjoy as well as a greatest hits compilation of ideas from my older campaigns. During a time of holiday shut down I was watching some older movies from my collection that I knew what I had to do with the Yuan-Ti.
I had the Cult of Grax representing one of the forces from one of my old campaigns. I definitely planned to include them in Toldara. While watching the 1982 Conan film and seeing Thulsa Doom, the cultists, and the snakes brought all this stuff together. The Cult of Grax would be a snake cult, their symbol would be a globe with a snake coiled around it, they would have custom Clerics, and this hybrid snake & man figure would represent an especially devoted Graxian Cultist rewarded with dark powers by their dark lord.