The Yuan-Ti - Snakes in the Grass

The Yuan-Ti are are a species of many different forms & castes. Originally human, or originally human-like at least, during the times of Hyperborea, the Yuan-Ti lived in cities & temples in the jungles of the world. There they worshipped the gods Seth & Apep, and believed they were superior to everyone else. Their nation flourished and started to expand taking in more territory. They rose after the Dragon / Giant wars, and most other species were beneath them, only good for slaves and sacrifices. If the conquered another nation they left the basic infrastructure active, but plundered any temple and noble house replacing it with their own. Eventually they were shown the way carry out dark rituals, blood sacrifice, and cannibalism to change into shapes that they admired, that of snakes. Many changed according to their caste that still bears the same until this day.
When Hyperborea split and the continents came to where they are today, the Yuan-Ti were scattered along with their jungles, and cities. Each city had a temple at it's heart but the two most powerful sank beneath the waves, one far from what is known at present of the coast of China and was referred to Mu. Another is off the coast of East Africa and called Lemuria. The casts have remained, with slaves at the bottom, Broodguard above them, Purebloods above those, Malisons above , with the Abominations & Anathema at the highest rank.
Live young is rarely born, and most are born from clutches of eggs. When two mate they are usually of the same caste, as the child will always follow after the lowest caste level. Purebloods are the most common with only slaves outnumbering them in some city states. As for the highest ranks, they rule with an iron fist, and are more snake than human, as well as being very rare. Yuan-Ti believe that once ruled the world and will do so again given time, and when they have the numbers.

"Evil, absolutely evil. No other way to describe these creatures. My people knew of them for millennia, and thought most of it was myth. Now we know better and now yours will know too. Now that the white flying serpent in the north now moves to claim the tomb of the black serpent for himself, this will only incentivise the rest of their kind to start exploring more, and to invade Europe after a time.
It doesn't matter if they can't stand the cold, they will come. That bitch, Elonga Lola, she is the first known and likely the one that's furthest north. They are probably already heavily embedded in cultures around the Mediterranean Sea without anyone knowing. I was lucky I was sent after this one, at least I had one to help, even if he is a thief. At least Fleec, his name of course, will back me up and I'm sure he will be happy to see them gone too."
Martine Sascha, Aethiopean warrior & hunter, explaining the threat of the Yuan-Ti to the Parisius nobility.