The Razor's Edge
The Elves of Greyhawk were originally descended from the Eladrin. More than 4000 years ago, the ancestors of the modern day Elves came through to the Prime Material Plane from the Feywild. This crossing over was unexpected and caused by magical experimentation ( will be detailed in the future ).
While they lead long lives and document their history in songs, verse, and scrolls, the memory of the “Crossing” has not only been forgotten, but it was never documented. It’s as if there was something keeping them from documenting and remembering the event.