The Rise...

Far in the east, the greatest Mordonian tribe – the Falenians – conquered the indigenous people and migrants alike.  In time, their kingdom – Falena – conquered the entirety of eastern Avernos.  The Falenian leader was crowned Overking and decreed that his land should henceforth be known as the Great Kingdom.  Falena declared the birth of a new calendar, and with the Declaration of Universal Peace, the sun arose in the east on the first day of the first Common Year.

...and the Fall

Eventually, the Falenian kings grew lax, caring more for local prestige and wealth than the affairs of distant vassals.  As rulers passed, each was replaced by one more dimwitted and incompetent, until the outer lands of Falena began to declare independence.  One after another, they broke away from the ineffectual Overkings, creating their own governments.  By the year 356 CY, the ruling dynasty had all but crumbled between the barbarian tribes from the north constantly raiding, and the last of the central lands declaring their independence.