The Yuan-Ti are a race of snakelike peoples who attacked the village of Lassibar.
Through discussions and negotiations, they informed various party members that they have been ousted from their home by a group of people who sound very similar to Sinkers. While they attempted to gain help from the people of the Material Plane for assistance, they were denied due to their looks and many of them killed as a result.
With nothing left but subterfuge, they caused the mysterious disappearance of villagers in Lassibar to attempt to draw good adventurers in, and then force them into a ritual that will change them into Yuan-Ti, thus simultaneously forcing and allowing them to assist.
Clearly a rather proud race originally, some of that still remains, but the very few survivors of their race have resorted to desperate and not too well thought out schemes to get their home back.
Their plan was less that successful, with many more of their number perishing. Only two survived the attack, one who was hours away in the forest and was spared by Kerys Nae Gwennen and Calairiel, and one who paniced, grabbed Surosar and teleported out to escape. The latter was later found dead at the hand of an ork.
In following leads, the party came across an abandoned pyramid temple whose décor seemed to be Yuan-Ti in nature. Close inspection revealed that it was perhaps something else in the past, but the Yuan-Ti had taken and written over it. A brief look into the lower layers confirmed this theory. Within the pyramid’s Yuan-Ti levels there was a portal to a demiplane called Coil, the true home of the Yuan-Ti, which opened when Khazmin Pyrrhus blindly activated a device and turned into a Yuan-Ti himself. However, despite this being the home of the Yuan-Ti, there were no Yuan-Ti in the temple to go home. The party rested there, noting such features as huge ziggurats and rivers made of snakes.
Nearby the pyramid was a tower belonging to The Sinkers, lending credence to the theory that the Yuan-Ti were driven out by The SInkers.