Yuan-ti are snake-human hybrids, created in the earliest days of civilization, whose culture fell from an advanced, enlightened state into fanaticism and cruelty. They live in a caste-bound society in which those who most closely resemble humans make up the lowest stratum, while the most snakelike constitute the highest and most powerful. One distinctive characteristic they all share is the innate ability to cast suggestion: like Kaa in The Jungle Book, they try to win your trust before they mess you up. Another is that they all have magic resistance, so they have no reason to fear spellcasters more than anyone else.
The most common and least powerful caste are the yuan-ti purebloods. (Counterintuitively, “pure” is a pejorative to the yuan-ti; the more adulterated by reptilian essence they are, the more they’re esteemed.) Their physical abilities are average-ish, with a slightly elevated Dexterity; their Intelligence and, particularly, Charisma are higher, implying a species that approaches combat from a mental angle first. This implication is emphasized further by their proficiency in Deception and Stealth. They have darkvision, suggesting that they’re most at home in dim places and/or most active at night. Along with suggestion, they can cast the cantrip poison spray three times per day (presumably only at its base damage level of 1d12—although they have 9 hit dice, there’s no mention of their spellcasting level). They can also cast animal friendship on snakes, for whatever that’s worth.
According to the Monster Manual flavor text, yuan-ti purebloods often put on cloaks and try to pass for human in order to “kidnap prisoners for interrogation and sacrifice,” so let’s start with that: The yuan-ti wants to kill you, but it doesn’t want to kill you right here and now. Instead, it wants to get you someplace where it can kill you in a way that makes its gods happy.
