The Great Dying

Summary

Eons ago, there lived the three races- Dwarves, Elves, and Orcs. The Elves were masters of magic, the Dwarves were fierce warriors who wandered wherever their sword took them, and the Orcs were the eldest of the races, venerated for their ancient wisdom and inventions. There were others, of course, the fearsome Dragons and the shrewd Beastmen, though they were considered to be among the races of Monsters, who tormented the three great races, hunting killing them. 
Everything changed, however, with the Great Dying. The plague wiped out untold numbers of every race, and skewed the sex ratio at birth, causing females to outnumber males by a wide scale, something that affects almost every race to this day. Elves adapted by removing males and females from the equation entirely; they became hermaphrodites who could both carry or sire offspring. Dwarf females gained the ability to transition to a male upon reaching maturity. Orcs, however, could not keep pace, especially with their slow maturation and reproductive rate, and a huge number of their population already perished. Though the Orcs died out, their remnants can be seen in the Mountain Elves, the offspring between ancient Elves and the Orcs.
Humans came after the Great Dying, though exactly how long after, no one can say. They were unaffected by the warped sex ratio at birth, and their males displayed the unusual ability to mate with females of any other race, and produce only children of the female’s race. Human men became prized by monstresses, who needed them to prevent the extinction of their races. Human men were kidnapped and enslaved by monster kind. and in retaliation, human women had to learn to protect their men.

Historical Basis

The myth is believed to be largely true, as it simply details how the current state of the world came to be. However, scholars believe that Elves have always been hermaphroditic, and at least for a very long time, they've not had two biological sexes.

Spread

It is remarkable in anthropological sense simply for how widespread it is across diverse cultures, races, and locations, albeit with embellishments depending on the version you hear. This version is one pieced together from multiple sources.

Date of Setting

Untold eons ago