Teyamut

Off the western coast of Kauvet, a stone's throw from the edge of The Redwillow Forest, there exists a pair of landmasses that form an overgrown island. It is populated by poisonous plants and dangerous creatures and is known for its large population of snakes. It is this island where the Yuan-Ti people are said to originate from.
In a time at least 1,000 years past, the dragon fiend Tiamat made her descent to her hellish lair, but on the journey lost one of her five heads. The green head landed on the island and was assumed to be another iteration of the areas many serpents.
The yuan-ti who discovered it worshipped it as a god, using magic to claim its power for themselves, inadvertently making themselves more draconic in nature. They would grow their numbers by performing similar rituals using actual serpents in the years to come. The jungles of the islands are infested with monsters whose ancestors also absorbed the power of Tiamat by drinking her blood as the head fell to earth. Despite all of this, the yuan-ti people hold a deep hatred for chromatic Dragons and anyone else who claims their snake god has any connection to the archdragon.
The capital city of yuan-ti culture is called Teyamut, named after a misunderstanding of their snake goddess' name. Over the years, a secret inner circle developed among the worshippers of her cult determined to prove once and for all whether the accusations of their god's identity were true by enacting a ritual that would reunite Teyamut with the other four heads of Tiamat.