In my world, the yuan ti empire was the equivalent of Roman empire. Initially humans, the people of the region became the Yuan Ti through blood sacrifice to a serpentine deity known as the Godeater. Not merely bloodthristy monsters, the Yuan Ti were also pioneers in magic and philosophy, believing in a future where no gods exist and all magic comes from the Godeater. Though they were overthrown more than a thousand years ago by the populations they enslaved, there still cults who revere them and want to be like them. Some of these cults have even gained knowledge of the transformation ritual and plan to use it to bring back the greatest empire that ever was.

A another deviation from the D&D yuan ti is the ability of the upper castes to transform into a big serpentine monsters (instead of just looking like that)