Yuan-Ti

Yuan-Ti are serpent like humanoid creatures that are all that remains of an ancient human empire that worshiped Dendar, The Night Serpent. 
Through profane rituals, the human cultists of Dendar, slowly transform themselves by mixing their flesh with that of snakes. This practice eventually created a caste based society of snake / human hybrids, in which those with the most snake like features became nobles and leaders, while those with more human features became lower cast workers, often used as spies or agents in foreign lands. 
One of the largest populations of Yaun-Ti in Wildemount existed in the Chultan city of Omu. Omu was a powerful conclave of Yuan-Ti and cultists of Dendar up until a century ago when they were mostly killed off in a failed attack against a rival city named Mezro.
The few Yuan-Ti that survived the counter-strike against Omu from Mezro, scattered into the jungles of Chult where they may still hide today. Occasionally, rumors of Yuan-Ti being spotted in the old ruins of Omu have surfaced, However, due to the extremely remote location of Omu's ruins, that is known to very few, these rumors have not been verified.