The Last Cure

Sisavah Solkash, the person who is considered a savior of the yaun-ti, was a mother whose son got scale rot, the cure wasn't safe to give them and involved ingesting a poisonous flower, the desert rose. The Cult of Mol'Sylk offered a "safe" way to take the cure, but it involved the devouring of an Anathema. They caught a pregnant Anathema, and ate of it's eggs, which they were told would help and heal them. The children of Sisavah, along with the children from the cult were afflicted first, their forms altered and changed. One of Sisavah's sons was turned into a Anathema himself. He was locked into caves deep in the mountain, for his protection. The children born that year were normal until they hit their 25th birthday, some of them were afflicted with the curse but none as much as Sisavah's family. Losing their form and cursing some of the future generations to become serpents, the blood of which courses through every brood that stems from the original tribe. The original Anathema was murdered with the thought that it would cure them but it wasn't a cure, it made their affliction permanent.  It's blood  is considered sacred, but is highly cursed.
Inside: The true tale about what Sisavah Solkash gave up so that the yaun-ti people could help those in need. Losing their form and cursing some of the future generations to become serpents, the blood of which courses through every brood that stems from the original tribe, and also there is a hint of a great deity, but the more you read the more it sounds like what you saw in the the temple, what Mukhfi made himself into. It's called a Anathema, and it's blood is considered sacred, but highly cursed. All Anathema were supposedly "destroyed", Sisavah saved one and placed it in a safe place away from the chaos, deep in a mountain. 
In Short: Big snake sleeping and being bled for blood, blood being mixed with magic and hallecignens. The Last Cure reflects the curse Sisavah put on the people to help them survive the scale rot that killed the founder of the Yaun-Ti.