In the beginning, the cosmos were empty, save for two planes, the Astral Sea and the Elemental Chaos. These planes were, and still are, vast. In fact, some say that they are infinite. Out of the Elemental Chaos though great entities were born. These entities were the primordials. Made of the pure elements, the primordials were masters of all things elemental, and could create as well as destroy. The primordials thrived on equal parts creation and destruction and as such, they endlessly created great works, only to destroy them almost as soon as they were completed.
One of these works though caught the attention of the other original denizens of the cosmos, the gods. The gods saw the primordials create the material plane (commonly referred to as “the world”) and its mirror planes, the shadowfell and the feywild. The gods looked upon the plane and saw pure beauty, not just in the plane itself, but also in the creatures that inhabited the planes. The gods alighted in the world and its mirror planes and walked among the beauty they found. The gods found a home away from their homes in the Astral Sea and soon fell in love with the world and its inhabitants. Thus were they horrified when the primordials sought to destroy the beautiful world that they had created. The gods, rather than see the world destroyed intervened and attacked the primordials, beginning the Dawn War.
None can say for sure how long the Dawn War raged for time had little meaning to the gods. Some say the war raged for decades while others claim it lasted centuries. While the duration of the war is not certain, what is certain is that the war was extremely costly for both sides. Gods and primordials alike were killed in large numbers. The perfect heaven in the Astral Sea that the gods had created was shattered — possibly forever — and many astral dominions were decimated. Although the war was costly, the gods believe that it was succesful — at least so far — in that they did ultimately defeat the primordials, and thus saved the world, but only at a high cost.
The primordials proved much tougher than most of the gods realized. One primordial in particular, Haemnaethuun, cut a large swath through the ranks of the gods and their followers. The exact toll is not known, but many sages believe that Haemnaethuun personally slew dozens of gods in single combat. In the end, it took the combined power of six gods to defeat Haemnaethuun, and even then they could not actually destroy him. Instead they had to content themselves with sealing Haemnaethuun’s heart behind six powerful seals. These seals though are far from impenetrable as has recently been shown and it is believed that the Heart can be used both as a powerful weapon and as a means by which to revive Haemnaethuun.
With Haemnaethuun defeated though, the primordials lost their leader and soon fell into disarray. The gods rallied behind the defeat of Haemnaethuun and soon had all the primordials either imprisoned or destroyed as far as they could tell. Undoubtedly, a few primordials were able to flee, but none have sought to challenge the power of the gods since.
By all estimates several millenia have passed since the end of the Dawn War. The gods though are thought to still not have recovered fully. It is said that the “heaven” of the Astral Sea is still shattered and that Erathis works tirelessly to try to repair it. Many gods were killed and several others had the domains they called home destroyed, forcing them to either share domains with others, or seek out new homes on new planes.
The other fallout of the Dawn War was the destruction of the Living Gate, which was said to have sealed off the Far Realm from the rest of the cosmos. With the Gate’s destruction, the aberrants of the Far Realm were able to stream through the opening and start populating the Astral Sea. The natives of the Far Realm have since proven to be almost as dangerous as the primordials. Aberrant forces seem bent on conquest and destruction. Furthermore, it seems that at least one Far Realm Entity, the One Who Hears is interested in retrieving the Heart of Haemnaethuun. What this entity’s plans for the Heart are is known only to it, but it is unlikely that anything good would come of it obtaining the Heart.