Zehir

Zehir
Snake in the shape of a dagger
Aliases	The Great Serpent, Master of Poisons and Shadows
Alignment	Chaotic Evil
Pantheon	Dawn War, Yuan-ti
Portfolio	Poison, Darkness
Domains	4E: Darkness, Poison 5E: Death, Trickery
Home Plane	Samaragd (Tytherion)
Worshippers	Yuan-Ti, Assassins

Zehir is an evil god of darkness, poison, snakes, assassins, and Yuan-Ti, mentioned in 4E and technically existent in 3.5/pre-spell plague. (Zehir means "poison" in Turkish).

He has a long-running conflict with Tiamat. Queen of dragons decided to make her realm in Zehir's home (Tytherion, the Endless Night) and started her conquest from the deep caverns that Zehir and his worshippers preferred. The war was a partial success but grinded to stalemate. So, now Zehir and his worshippers rule the mountains and fields, far above the depths they prefer to crawl in, while Tiamat rules the caverns below, far beneath the skies and mountains they prefer to fly through. It is possible that everybody in Tytherion would be happier with a simple trade, granting Tiamat the heights and Zehir the depths, but neither Tiamat nor Zehir want to negotiate peace (too proud and angry at each other) or launch the first attack (both keep preparing to defend against the opponent without actually fighting).

The second expansion pack of Neverwinter Nights 2 has him as the main enemy. There you break up his scheme to take over the Sword Coast. Oddly enough pre-release materials for the game indicate Zehir was supposed to be more prominent within 4E. It makes a big deal about Zehir being one of the new gods in 4E as though it was something people would care about.

Zehir himself shares many features in common with other serpent-related entities, such as Dendar, Merrshaulk, and Set. Like Dendar, Zehir is associated with darkness and wants to devour the sun. Zehir's realm and Merrshaulk's have similar names, and in the 4E supplement The Plane Above: Secrets of the Astral Sea it is stated that Merrshaulk dwells within a temple in Zehir's realm. Finally, various official materials have stated that Set was originally going to be the name of the 4E snake god until the alternate name Zehir was put forward.

As of the 5E Dungeon Master's Guide it appears that Zehir may have been an alias for Set all along.