Dungeons & Dragons 4th edition (2008-)
Zehir rules the mountains of Tytherion, the Endless Night, while Tiamat, Queen of Chromatic Dragons, rules the caverns below.
Eons ago, after realizing that she could not defeat Bahamut, Tiamat decided to obtain her own realm and invaded Tytherion, hoping to unseat Zehir. She won a partial victory, but could not defeat him entirely. Zehir and his serpentine worshippers prefer the deep caverns, so she attacked those first. The result is the strange division of Tytherion today; Zehir and his serpents rule the mountains and fields, far above the depths they prefer to crawl in, while Tiamat and her dragons rule the underground caverns, 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 God will admit such a desire; their pride is too great. Tiamat wants all of Tytherion, while Zehir regards her as an interloper.
While technically the two sides are still at war, they are currently not engaged in actual fighting; both sides are waiting for the other to strike first, and are bolstering their defenses in preparation for an attack. They have been waiting for eons to continue the war.
Zehir is an interloper deity who came to Toril shortly before the Spellplague but was a relatively unknown power until after that event. He has an affinity for snakes. After the Spellplague, Zehir, or at least part of him, was stranded in Realmspace; he dwells in the Towers of Night along with his rivals Sseth and Talona, and his superior, Shar.
Neverwinter Nights 2: Storm of Zehir
The plot of Neverwinter Nights 2: Storm of Zehir surrounds Zehir's plan to take control of the Chultan peninsula and the Sword Coast North. It describes his methods as aggressive, yet subtle. He likes his wishes carried out quietly, but boldly and does not forgive failure, though those who continue to please him can expect to be richly rewarded. He is confirmed as an interloper deity, though it is not confirmed where he is originally from, nor when he arrived on Toril.
In one of the last few scenarios of the game, the player characters defeat his herald, a powerfully-built giant yuan-ti abomination wearing an aztec-esque headdress, and then get the chance to converse with the deity, represented as two huge, red floating eyes set in scaly sockets and with feathery eyebrows. The voice that issues forth from this apparition is calm and confident. Despite the fact that this plan was ruined he reveals that it was but one of many schemes that he is sponsoring all over the world.