The Great Migration is a well-known historical event occurring over a thousand years ago. Historical evidence of the migration are difficult to find due to the turmoil of the period.
During the rising tensions between the Ancient Suel and Ancient Baklunish nations, thousands of refugees fled east away from the growing destruction. Fleeing east over the Crystalmist, The Oeridian people (who were a confederation of barbaric tribes in close proximity to the warring empires), took the wars as a sign to migrate eastward in search of their ultimate destiny.
They were the first large group to enter the lands of the Flan, which they termed the Flanaess. Meanwhile, The conflict behind them culminated in the legendary Twin Cataclysms which laid waste to both empires and changed the face of Oerth forever. Survivors from both shattered empires also joined the migration, spreading across the face of the Flanaess.