War of the Gods
The War of the Gods was fought in the long past. So long it predates the Cataclysm by thousands of years. It exists mostly in the story of the gods as their first challenge when they turned their eyes to the world of Celestia. There had already existed powerful beings that lived on Celestia and who had subjugated the first dragons and several other races that were fragments of the gods. They were are known as the Ashuri.
The Ashuri and the gods fought, part in the materials world and part within other planes of existence. Time itself broke from the conflict, so any guess as to how long this went on is impossible to say. The skies churned, the earth shook, the oceans boiled, like and death itself went wild. When things returned to calm the Ashuri were destroyed. The Gods themselves hadn’t gone unharmed and in the end the remaining deities were sundered into two: The Gods of the Wheel and and the Gods of Corruption or rather Demons. Order and Chaos personified.