A yuan-ti child -- a pureblood who looks entirely human except for some very subtle snakelike characteristics--is found alone in a jungle ruin by a passing (and heavily-guarded) caravan of traders who harvest a rare addictive flower from the jungle for trade in a nearby city.
The head of the caravan takes in the child, not realizing its inhuman origins. The child grows up with the caravan believing herself to be human and learning both tradecraft and how to survive in the jungle, but she begins to have dreams of snakes and other racial-memory things, possibly even visions from a deity. After forcing her mother to admit she's adopted, she runs away from home to seek out her origins, but she is pursued by her adopted human "brother" who wants to bring her home.
They find the ruins where she was discovered as a child, and she discovers her true nature. She's horrified, but also enraged when she discovers denizens of the Underdark stole away the other yuan-ti from her settlement, including her real family, to use as slaves. She ventures into the Underdark to try and rescue any survivors from her "real" family.