The Age of Rebellion

The Age of Rebellion was the age before the The Age of Mortality. During this age, people everywhere began to rise up in rebellion of various kinds, ranging from teenage rebellion against their parents all the way up to mortals declaring war against the gods themselves. This eventually culminated in a cataclysmic disaster known as The Shattering.
Ostensibly, reasons for this rebellion varied wildly. It took over thirty years after Rebellion Itself arrived in The multiverse for anyone to realize that this was the result of some external entity. Even then, this knowledge was a tightly guarded secret even among gods. Only the rulers of the four primary elemental planes knew that anything was amiss, which is why they were targeted first when The Shattering began.
The Avatar was a tree located on top of a hill in Bytopia known as The Hanging Tree. The Dukes of Air learned the truth, but the rulers of Bytopia were suspicious, and the dukes had to slay the current rulers in order to gain entry. They crafted a special spear to do this, known as Vel, and were successful in slaying the existing rulers.
Unfortunately, saving the world proved far more costly than the Dukes of Air had predicted. Firstly, cutting down The Hanging Tree did not break the spell that had been placed upon the mortal populace.
Secondly, it also accelerated the coming of the next age. The next age was the The Age of Mortality, and the moment it arrived the Dukes of Air were rendered into mortal beings. In addition, the spear they used to do the deed had vanished. The spear was later found again, in possession of the new avatar Iapetus, who imbued it with even more power than it originally had, christened it Dénouement, and used it to attack Bahamut.
When they realized what they had done, the Dukes of Air led a charge into Celestia to banish Iapetus to the plane of Vigrid. They then took Dénouement and sealed it away into a small demiplane of their own creation. This demiplane was later brought into Sigil when Vun Roade planted magic beans in the Temple of the Four Winds during Session 13: Tornac's Bad Day.