Titanomachy

In Arkhosian mythology, the Titanomachy was a series of battles fought across much of Alterra, consisting of the Titans (an older generation of gods) fighting against the Celestials (the younger generation of gods, who would come to reign on Mount Celest) and their allies. This event is also known as the War of the Titans, the War of the Gods, or just the Titan War. The war was fought to decide which generation of gods would have dominion over the universe, and ended in victory for the Celestial gods.

Background
The stage for the Titanomachy was set after the youngest of the Titans, IO, the dragon of Time, overthrew Primordial Chaos at the beginning of the Mythic Age. During their battle, Primordial Chaos wounded IO with the blade called Entropy, which would eventually lead to IO's dissolution.

IO took his father's throne after dispatching Primordial Chaos, becoming the King of the Titans, siring three children by his wife, Mystra, the Titan of Magic: Arakam. Albion, and Thallasus. It was not to last, though, for the wound Entropy left on him only would only grow worse. It began to corrupt Time as it worsened, splitting and fracturing the linear order of time as IO succumbed to the poison of Entropy. These fractures of linear time are referred to as Dragon Breaks, and are the result of the effects of Entropy on IO, and thus on time itself.

In order to prevent time from fully unraveling as Entropy worsened, IO fractured himself into two beings, the platinum dragon Bahamat and five-headed Tiamat, who he decreed would be his heirs and rule in his stead. This proclamation would fall on deaf ears. Freed from IO's rule, the Titans, led by the Titan of Want, sought to rule the universe in their own right, refusing to recognize the rule of the new gods. Mystra, with her children, fled the wrath of the Titans and went into hiding.

Mystra would then bring her children to a cave in the Silios Mountains, where they would be raised in defiance of the Titans.

Bahamat and his siblings conquer the universe
Upon reaching adulthood, Bahamat set out to wage a war against the titans, with his siblings as his allies: Tiamat, Thallasus, Albion, and Arakam. In addition to these, Bahamat convinced Polydegmon, IO's champion and eternal lieutenant, to join him and his siblings in retaking the throne.

Fighting on the other side were the Titans: the Titan of Want as their head, with the Elemental Titans of Water (Rath), Earth (Takesh), Fire (Su'gra), and Air (Vasah) forming their vanguard. Other important members of the Titans' army were Hellion, the Titan of Blood, Annyx, the Titan of Night, and Korrel Nasha, the Titan of Memory.

According to legend, the war would last a hundred years. During this war, Albion began forging for her siblings the first of the Dragon's Teeth as weapons of war to beat back the Titans, crafting the Lightning Blade for Bahamat, the Cruorwhip for Tiamat, the Steadfast Gauntlets for Thallasus, and the Harvest Sickle for Arakam. Eventually, Bahamat and the other Celestials won their victory, imprisoning the Titans within Oblivion. For their actions as the Titans' vanguard, special prisons were constructed for the Elemental Titans: what would eventually become the Elemental Planes, designed and built by Albion to hold the Titans for all eternity. It is said that when the Titan of Want saw her war was lost, she descended upon Korrel Nasha and feasted on his flesh and soul, absorbing his essence into herself before disappearing, never to be seen again. In the aftermath of that final battle, Bahamat decreed that the name of the Titan of Want would be stripped from her, never to be spoken again, lest its utterance summon her back from whence she had fled.

Following their final victory, the Celestials divided the world amongst themselves: Bahamat, as the rightful heir of IO, was granted dominion of the sky and air, and was recognized as the overlord and king of the gods. Tiamat, her twin's mirror, was granted the world below, the Underdark, and the realms of the dead, tasked as the eternal jailer of the Titans chained in Oblivion. Thallasus, in turn, was granted the sea and all the waters. Each of the other gods was allotted powers according to the nature and proclivities of each. The earth was left common to all to do as they pleased.