Note: I am mostly using the history and world of Golarion from the Pathfinder RPG. I have made a few changes, according to my original design.
The world of Golarion was changed forever when a large meteorite crashed into it. This has been called the Sundering and Earthfall, interchangeably. This cataclysmic event shattered continents, destroyed nations, flooded lands, and caused terrible destruction.
A thousand years of darkness followed, during which Orcs and Dwarves emerged from underground to populate the lands above. Near the end of this age of darkness new human kingdoms were formed, and the elves returned to the land through portals they had setup before the Sundering. Strangely, the elves had no recollection of their time before.