The evil, snake-people of the steamy jungles in Metzono and Hezhz. These folk will take up yer children as quick as look at yer.
— Captain Sholm of Fort Hope in Metzono

  The once-human creatures known as yuan-ti inhabit the dilapidated temples and building of their empire long, long ago. They rarely strike out with any force, preferring to sneak and subterfuge. They kidnap their victims to sacrifice to their deity, Zehir.  

Once-humans

During a time of great calamity the Yuant Empire rose and worshipped the Evil snake deities. With their deities they ruled with ruthless power, enslaving those deemed "impure." Eventually, the emperor partook the blood and flesh of a divine snake and grew aspects of it. His followers did the same, and made everyone else as well. Unfortunately, this erased their minds, turning them all into yuan-ti.  

Fanatics

These new yuan-ti were more ruthless than before, and began sacrificing their slaves to their serpent deities. Eventually the empire was defeated in the Serpent Wars. The defeated yuan-ti retreated to their more remote places and built themselves up. Now, they strike out from shadows and snatch unfortunate passersby from secluded or heavily forested roads and homesteads. While sole persons are usually taken, it is not unheard of for the yuan-ti to take a whole home.  

"Seething Lizards!"

Yuan-ti see their usually-neighboring Lizardfolk as less-than, and an affront to Zehir. They believe they would make excellent slaves and servants to their dark deity. They violently bemoan of the lizardfolk's inability to worship Zehir, torturing captured scouts and collecting their blood in rituals. They often call them "seethers" or "seething lizards" due to their hatred of yuan-ti, a thing the yuan-ti believe to be misplaced.  

Pragmatic Liars

In a cruel, fitting joke, the yuan-ti are cold-blooded, having no warmth for their previously human emotions. They see such things as weaknesses, feigning pain or injury or weakness to draw out the compassion of others. Their deception affects travelers who believe their stories of riches in the hills for "simple services of labor." Needless to say, there are hardly any locals who have any mercy for yuan-ti.  

Religion

In yuan-ti folklore, the deity Zehir devoured the other serpent deities, making him chief and only. This greatly impressed, the yuan-ti who emulated this behavior, literally slaking their thirst of power. They did not know, though, that their hunger was not slaked, but whetted. The yuan-ti also do not worship for love, but power.