Zehir is the goddess of poison and darkness. 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.