Zehir

Zehir, the Cloaked Serpent is an evil god of poisons, assassins, and darkness. He is the creator of snakes and also of yuan-ti, but most of them were annihilated during the Calamity, and those who survived are in self-induced stasis or hunted by the followers of the Spider Queen and the Crawling King.

Appearance
Many depictions of him can be found in his long forgotten and abandoned temples. He takes the form of man's body with a half-dozen arms and a gargantuan fanged serpent head, with strands of thick, dark hair sprouting from his scaled body that swirl around to form layers of shadow, obscuring his entire form.

Religion
The tenets of Zehir's religion are largely related to the sowing of chaos. He believes that to keep your acts obfuscated and secret is the greatest means of success. He commands that his followers strike quickly and without reason, and blind the target with their own confusion. He commands his followers kill slowly, agonizingly, or worse, make them enjoy it.

The Vault of Shumas
In the Aggrad Mountains of Marquet, there was a hidden temple guarded by yuan-ti, a manticore, and an undead spirit calling herself "the caregiver". The caregiver watched over the sleeping children of Zehir "until our epoch begins."

Urukayxl
On the island of Urukayxl in the Lucidian Ocean, there is a large five-sided pyramid that serves as a temple to Zehir. The pyramid and its surrounding village were largely populated by yuan-ti.

Relationships
The Cloaked Serpent loathes the Lawbearer and the Wildmother, as they stand for everything he despises: love, order, and life. His followers try to appease him by using poison and fire to undermine civilization and consume nature.

Uk'otoa
The Cloaked Serpent is the creator of Uk'otoa, the Great Leviathan. After Zehir was banished during the Calamity, Uk'otoa was left to rule the Lucidian Ocean as his proxy. However, Uk'otoa began demanding that people who entered his domain renounce their gods and worship him, which enraged Zehir. To punish Uk'otoa's insolence, in the fifth century PD it was cultists of Zehir who corrupted a series of temples dedicated to Uk'otoa and sealed the leviathan deep beneath the ocean floor.

In one of these temples on Urukayxl, the Mighty Nein saw three murals depicting Uk'otoa, Desirat, and Quajath. The three lesser idols have a pact; apparently, if one escapes, it will help the others, so presumably Zehir is an antagonist of Desirat and Quajath as well.

Trivia

 * Matt confirmed in that the three entities pictured in the yuan-ti temple, including Uk'otoa, are all creations of the gods. He said some creations of the gods became "pseudo-gods" after being left behind by their creators.
 * "Zehir" means "venom, poison" in Turkish.