Revolution
The clash between the gods that divided them into The Nine and The Seven. The betrayal by Gruumsh and Lolth began it. The birth of Pelor finished it.
Gruumsh's rage at the rigging of the lot by Corellon seethed, and he plotted against the other gods. He was approached by the young goddess Lolth, who needed an army to stand against her father and his Fey creations. Gruumsh led the Orcs in war, distracting the gods while Lolth spun her plots.
There were other gods with their own grudges to settle. Asmodeus led the devil legions forth from The Nine Hells, seeking to topple Erathis and Moradin from The Clockwork Nirvana of Mechanus and The Peacable Kingdoms of Arcadia respectively. Torog chased Avandra across the spheres until the Changebringer traded some of her luck to Moradin in exchange for dropping Mount Celestia on Torog's head. The Jailer was imprisoned deep in the roots of the great mountain and eventually wandered through the spheres to the Material Plane. Deep in the Underdark he built his domain, The King's Highway. With Avandra's luck, the Builder and his dwarven armies turned the devils from Arcadia. Erathis was forced to flee Mechanus ahead of Asmodeus's armies. As the bodies piled up, Orcus and the demons of The Infinite Layers of the Abyss joined the fray, turning the armies of good against their creators.
Meanwhile, Corellon and Gruumsh met on the field of The Heroic Domains of Ysgard, amid waves of battling Fey and Orcs. The blood Corellon shed gave birth to the first Eladrin, who joined the fight for their creator and seemed to turn the tide for a time. Corellon dealt Gruumsh a mighty blow, taking his eye. But Lolth hatched her plots, attempting to strike straight for her father's heart through her follower Zehir. She succeeded in a way—her assassin's blade struck her sister Senaera instead. The poison made Senaera deathly ill, causing Corellon and her mother Melora to retreat and pull their forces from the field. The divine essence released suffused Zehir's body, raising him to godhood.
Corellon sent out many heroes in search of a cure. One of these was Moradin's demigod son Kord, who journeyed through The Windswept Depths of Pandemonium, and The Ever-Changing Chaos of Limbo. Instead, he found a nest six of giant eggs unlike any known. He took the eggs from their sheltered grove, but in the roiling soup of possibility that is Limbo, five of the eggs fused into one multicolored egg. Kord took the now two eggs to the slopes of Mount Celestia, where the final battle was raging as the combined forces of the Hells and the Abyss assailed the last bastion of the gods of good.
Ioun had disappeared long before the outset of the war, leaving only a prophecy that hinted at the forces of good triumphing. Following the prophecy, Kord placed the eggs in the Celestial Flame at the peak of the mountain. But nothing happened. Frustrated, Kord left the eggs in the fire and returned to the battles below. The darkest hour, when the armies of Asmodeus, Orcus, and Gruumsh seemed about to sweep over all the spheres, presaged The Birth of the Dawnfather. Pelor rose over the battlefield in his shining chariot at the head of an army of celestials. The flame at the peak of Mount Celestia flared to greet the sun, and the spheres shuddered and suddenly began to turn as time raced forward. Sources differ on whether Ioun gave birth to Pelor herself or whether she merely found the place where he was born and raised him until he was ready to take his place in the firmament.
The young god rallied the forces of The Eight and drove the forces of evil back to their appointed realms. Two dragons burst from the Celestial Flame, one the platinum Bahamut, the other the many-headed Tiamat, chasing each other across the swiftly spinning multiverse.
A team of heroes led by Pelor, Bahamut, and Kord captured Gruumsh on the edge of The Bleak Eternity of Gehenna and brought him back to Mount Celestia for judgment. The Nine gods of good sealed their pact to one another at The Judgment of the Warmaker. Lolth delivered her final blow through The Sundering marking the dawn of the Age of Light and the end of the Dawn War.