Everyone knows about the Demonwar, when Devil, Drow, and Dragon encircled Greyhawk and laid siege to it for 51 years. But humans are short-lived, perhaps mercifully so, and so the horrors witnessed firsthand during the Apocalypse have since passed out of living memory. Only the eldest among humans could have seen combat during the war. Perhaps they were there for the final push at the Battle of Unity.
Yes, the battle of Unity. When the Five Races and assorted creatures numbered among the Forces of Good notoriously beat back the demon hordes all the way to their own portal, shielded by an obsidian dome in the middle of the desert. As the retreating demon army slowly used the portal to return to their hellish plane, the forces of Good threw everything they had at that midnight shell. From the oldest dwarfish magics to that curious and loud new artifice, the human Cannon, nothing could breach that jet-black barrier… until someone turned a fragment of the demons' own magic against them.
The strangers brought forth a small black bead imbued with the essence of Annihilation; a spoil hard-won in the struggle against the demon army's supporters. No one was sure what to expect, but it was worth a try. They were careful. They practiced hitting the side of the dome with small rocks fired from a trebuchet. When they were confident they could hit fairly frequently, they placed the vile black egg into the center of the siege engine, hoped for the best, and fired.
The pillar of flame roared hundreds of feet into the sky. It is said all who looked upon the explosion felt the same epiphany. For when they saw what they had done, they realized that this day… this moment… was the final and definite end of the era known as the Apocalypse.
To cleanse the taint of the dead demons from the land, all the army's clerics came together and, upending an Everfull Pitcher, sanctified and purified the spot where the portal had stood, incidentally creating a lake of holy water in the process.
Then someone said it was a shame to create this purified lake in the middle of nowhere, and wouldn't it make more sense to put a town around it. For some reason, the idea spread like wildfire through the assembled forces. Some had no proper home to return to, for it had been destroyed during the war. Others had formed unbreakable bonds of camaraderie with the most unlikely of allies, such as elves and dwarves who'd fought side-by-side for the last 50 years, and wished to settle the same land together. Thus it was that humans, dwarves, elves, gnomes, halflings, and one Bronze Dragon founded a new City. A city in the middle of the desert wasteland. A City whose creed is All Are Welcome. The City… of Unity!