Zehir

 is an evil god of poisons, assassins, and darkness. He is the creator of snakes and also of serpent-kin, 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.

Urukaxl
On the island of Urukaxl 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.

Unknown Entities
In the Yuan-Ti temple on Urukaxl, the Mighty Nein saw three murals - depicting a giant purple bird with fire-like wings and three eyes of onyx, a gigantic sea serpent emerging from the water, with many small eyes of amber and three large eyes, and a worm-like creature with three eyes of ivory inside its mouth, ploughing through the earth. It is implied that they, like Uk'otoa, were created by the Cloaked Serpent.

Trivia

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