The Teija (Tey-ja)

The Teija people lived within the central area of Dunja during The Era of Tranquility. They were mostly human and half elf. They were lucky enough to live in an area mostly undamaged during The First Great Turbulence, in a settlement called Lissommoor and while they suffered hardships from the loss of trading partners and political allies, they were mostly able to continue their way of life, on undamaged and little changed land. They were occasionally attacked by scavenging groups and military powers attempting to seize good land during the chaos of the First Turbulence, but they had several lines of powerful sorcerers, as well as a strong militia, which was usually enough to disssuade attackers.
They worshipped Thabaci, who gifted her powers to their sorcerers. 
Jealous Neighbours About 8 FC, land neighbourings the Teija's was claimed by Lord Greywalk, a powerful figure who used the chaos of The First Great Turbulence to expand his territory and political influence. He was jealous of the beautiful, fruitful land farmed and lived on by the Teija, and wanted it for himself. However it was well-known that the Teija had no interest in moving and had a deep connection to the land, which they nurtured carefully. He sent a few envoys regardless, with what he felt were fair offers, but they were of course rejected. And he began to consider, with the encouragement of several of his more cruel and ambitious advisors, darker means of acquiring the land. One of these advisors had a number of expert assassins in their employ, and discussed means of dispatching the Teija with them.
One of these assassins made their way to lissommoor and to Goldheart Lake, which fed into the underground rivers that supplied the wells of Lissommoor. They emptied a single vial of extremely deadly poison into the lake, and immediately teleported away. Several hours later, a heavy green fog began to rise rapidly from the wells, and pervaded the settlement, poisoning the residents.
Thabaci heard the screams and prayers of her worshippers as the poison took effect and they began to die, and acted. Her snakes burst from the bushes and burrows around the village, and began to bite each resident. Their venom, infused with Thabaci's divine magic, changed the people into the first yuan-ti, ensuring their survival and giving them even greater powers.
When Lord Greywalk sent scouts to determine if the Teija had been destroyed, they were quickly killed, and the Teija rapidly avenged themself on Lord Greywalk. They infiltrated his town, Rootcall killed him and his top advisors, and claimed his lands for themselves. Their new snake-like visages and poison-spray attacks scared the soldiers so badly that most wouldn't fight.
While the Teija remained rooted in Lissommoor, they gradually consolidated their control over all of the former Lord Greywalk's lands. The rule became a theocracy, with the Teija enforcing worship of Thabaci.
While it is rumoured that some of the new Yuan-ti resented or were appalled by their transformation and left their people to make their own way, most of them embraced it, keen to never risk the kind of cruel attack like Lord Greywalk's ever again.