shiehalla

The Yuan-Ti worship the snake deity Shiehalla, a titanic serpent-like creature with a cloven head. In a terrifying display, this head splits into four pieces, acting as a collar framing the deity's face. It's body is formed of two conjoined tails adorned with a multitude of human arms that stretch out from its length. When the creature opens it's jaw wide, a ball of pulsing flesh is revealed with an ethereal face hovering inside like a ghostly brand. Those who have seen the face of Shiehalla have lost their minds entirely...or been transformed into spitting, hissing half-serpent creatures themselves.
Shiehalla is said to have been born of a vast serpent from the jungles of Balobia. Hatched from a corrupted egg or altered by a malevolent God, the aberration was so strong that the creature within became a god itself. Being the first snake deity Shiehalla had no followers at first. Many assumed it was a monster and rightly feared it as it roamed the jungles of Balobia consuming anything in its path with an unquenchable hunger. Even the demons that roamed the lands were taken into its jaws and there were tales that even lesser demi-gods were consumed in it's insatiable hunger.
Shiehalla still craved more than food its stomach, the flesh of lesser gods and demons providing no satisfaction. Weaving its way into the minds of man, God's proud creation changed its victims into snake-like humanoids. The Yuann-ti were born and not long after temples and civilizations of Yuan-ti were established. Shiehalla had everything it wanted and had left it's hideous mark on the old world until the Gods decided to put an end to this horror. They went to war with the creature and eventually trapped Shiehalla in a tear between worlds, saving man from its hunger. Yet the Yuan-ti can still be corrupted by the creature, though their form is less snake-like than before and their bodies are now considered to be that of the "pureblood". To the Yuan-ti it is an overwhelming honor to become an abomination and this fuels their sacrifices to Shiehalla, ever driven to create more of their kind to serve him.