The serpent creatures known as yuan-ti are all that remains of an ancient, decadent human empire. Ages ago their dark gods taught them profane, cannibalistic rituals to mix their flesh with that of snakes, producing a caste-based society of hybrids in which the most snakelike are the leaders and the most humanlike are spies and agents in foreign lands.
Humans Transformed
The people who became yuan-ti were one of the original human civilizations. Their society built great temples of stone and forged metal into armor, tools, and weapons. In their ceremonies they paid homage to the snake as the embodiment of the qualities they most appreciated. They developed a philosophy of separating emotion from intellectual pursuits, allowing them to focus their energy on personal advancement and expanding their territory. They believed themselves to be the most enlightened mortals in the world, and in their hubris they sought to become ever greater.
The serpent gods of the primordial world heeded the prayers of these people and hissed dark demands into their ears. The people tainted their souls by performing human sacrifices in the name of the gods, debased their flesh by cannibalizing their victims, and then performed a sorcerous ritual while writhing in pools filled with living snakes that enabled them to mix their flesh with that of serpents, becoming like the gods in body, thought, and emotion. Freed from the limitations of their human bodies, the yuan-ti used their new abilities to conquer new lands and expand their borders.