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.
https://forgottenrealms.fandom.com/wiki/Yuan-ti_pureblood