Cheliax started out as the vast, undersettled western provinces of Taldor; as Taldor declined, it became the intellectual and economic center of the empire, and then became independent, and then became an empire in its own right. Its patron deity was of course Aroden, the god of humans, prophecied to someday return and usher in an Age of Glory. Cheliax expanded north as far as Varisia and the Hold of Belkzen, became the world's largest naval power, annexed neighbor after neighbor, often peacefully.
The prophecied year of Aroden's return was 4606 A.R.. It was foretold that Aroden would bring an age of glory to the whole world, but Cheliax was where he would start, and the people of Cheliax eagerly awaited him. Religious orders swelled. The King prepared to abdicate. On the prophecied day there was a national festival.
And - no one knows for sure what happened, of course, but the general understanding is that the gods went to war. In locations as far as three thousand miles from Cheliax, three weeks of furious storms are recorded. The ground shook. At least five cities were swallowed by earthquakes, and at least two countries drowned beneath the waves as the storms receded from most of the world to form a permanent hurricane of unfathomable force and scale where the nations of Lirgen and Yamasa used to be. A hole ripped open between this plane and the Abyssal one; it is called the Worldwound and it is still open, despite the armies of five nations fighting to hold back the tide of demons.
After three weeks of this Aroden's clerics stopped getting spells from him.
Prophecy stopped working -- even, it is said, for the gods.
And Cheliax, built on the church of Aroden for healing, for clean water, for administration, for the authority of the monarchy, collapsed into a bloody and vicious civil war that lasted more than twenty years. A third of its population was dead by the time a claimant backed by Hell secured the throne and announced the nation would now worship Asmodeus.