Merrshaulk

Sseth (also known as Merrshaulk) is the chief god of the reptilian yuan-ti, a chaotic evil mastermind of traps, poison, and murder. His clerics are known as vrael olo. His symbol is a flying snake with bared fangs.

History
Like all the other gods of Toril, during the Time of Troubles Sseth was cast to the Prime by Lord Ao. He appeared over the jungles of Chult as an enormous magic-using winged snake, exhorting his priests to gather their strength. But as he slept in his lair, he was bound by the Mulhorandi god Set into a magical captivity in the Abyss; the Father of Jackals usurped his portfolio and continues to grant spells in Sseth's name (using his own Air, Darkness, Evil, Hatred, Law, Magic, and Scalykind domains).

The vast majority of yuan-ti are unaware of this; those know something are vaguely aware that the Slitherer Supreme is distressed in some way. Set's binding upon the yuan-ti god is imperfect, and fragments of nightmares sometimes appear in visions to his followers. Those who have seen them have generally interpreted the dreams to mean that Sseth is upset or angry. Repeated divinations have determined that Sseth is in distress of some sort, prompting many yuan-ti to quest for answers.

A small minority of Sseth's followers have learned a partial truth, that their god is losing a battle with another, and that the Okothian sarrukh are to blame. Those who know this are keeping it quiet, hoping to gather more information on the matter before revealing it. The Okothian sarrukh know the full truth, but are sowing disinformation among the yuan-ti to delay and confuse any investigation.

If Sseth is freed, he will likely declare war upon the Mulhorandi god, and (possibly) ally himself temporarily with Set's foes.