The Dawn War was the conflict between the gods and primordials at the dawn of creation. It lasted for untold eons and its effects are still being felt. The war ended when a particularly destructive battle destroyed the Living Gate, a seal that locked reality away from the alien Far Realm. With the breaking of that seal the differences and grievances of the gods and primordials were forgotten, they had a common enemy to face. It is thought that Moradin and Pelor sought the help of the primordials Sunnis and Zaaman-Rul in an effort to stem the tide of alien horrors pouring forth from the Far Realm. The current structure of the universe is a direct result of the solution that they devised.
The gods agreed to expend an unthinkable amount of their own divine energies to rework the shattered Lattice of Heaven (a cosmic framework that they had constructed prior to the start of the Dawn War to order reality) into a simulacrum of the Living Gate. They bound their divine power into focal anchor points and the connecting webs between them. These anchor points were located at the worst of the tears in reality, places where eldritch horrors of such vast intellect and size that they were direct threats to the gods and primordials were trying to cross over into reality. The primordials agreed to provide the raw material, the matter, of this new Living Gate. They bound earth, air, water, and especially fire to the focal locations throughout the universe. Thus were the stars and the systems of motes that orbit them born.
The Aftermath
In the wake of this colossal expenditure of power, neither side could continue the original conflict. The gods moved to the core of this new universe and each claimed for themselves one of the greatest stars as a dominion. The primordials journeyed to the outskirts of the universe, gathering to themselves the leftover material of creation in vast elemental nebulae. Both sides were greatly reduced in power. The gods could not access the majority of their divine strength, tied up as it was in the structure of the new Living Gate. Instead, they chose to commune with the Living Gate and attempt to exert influence over their chosen spheres by subtle alteration of its structure over time. Thus were they removed from any direct influence or interaction with the universe or its newborn mortal populations.
The primordials, always more numerous and less individually powerful than the gods retreated into their newly formed homes on the outskirts of the universe and tried to regain as much of their strength as possible. The expenditure of so much of the elemental material of creation on the Living Gate left them unable to easily rebuild themselves. Many of the more warlike and chaotic primordials began to turn on their fellows, leading the remaining elemental armies in bids for conquest. These massive elemental wars still rage on and off to this day, though only an immortal could tell who, if anyone, is winning.
The Exarchs Are Born
No one is sure exactly how or when the first of the exarchs were created. It is believed by many scholars that the Archdevil Mephistopheles, Lord of the Eighth Circle of Baator was the first that arose. The commonly held theory is that Mephistopheles was able to use some of the pacts and promises that he had made with his patron god during the Dawn War to raise himself above his fellows. Additionally, pacts made with mortal followers gave the archdevil a source of power in the souls and beliefs of those mortals. Other powerful devils recognized the value of this tactic and began to jockey for position among the hierarchy of Baator.
The greatest servants of the other gods also realized that the mortals of the universe were both a source of strength to be used and a valuable resource to be protected. Each group of future exarchs began to grow in power according to the temperament and values of their patron deity. The servants of Bane gained power through mortal conflict, and Erathis's exarchs through the establishment of civilizations and ordered populations. Some gods, such as Gruumsh, had groups of exarchs that gained power directly from a small, but dedicated subset of mortals.
It was during this early period in the cosmos that the illithid empire of Nihilath rose up and conquered a huge swath of creation. It is widely believed among modern scholars that this crushing alien empire was one of the reasons that belief in the exarchs as the voices and hands of the gods grew as widely and rapidly as it did.