Zehir

Zehir is an evil god of darkness, poison, snakes, assassins, and Yuan-Ti, mentioned in 4E. (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).