Drow

As the High Elves dug for stone for their more modern cities, they soon found the Below, rich with resources both magical and mundane, they sent groups down to form outposts and to gather such resources. 
Over the years, these outposts became deeper and deeper, and a High Elf mage named Julinee imbued a group of High Elves with abilities to aid them for surving the perils of the below, enhancing their darkvision and enabling a better natural skin tone for camouflage. This was the first instance of the creation of the Drow. Although Julinee claims to have completed this feat alone - some theorise this was actually heavily supported by that of a Deity. 
The Drow as they delved deeper, eventually discovered the Quaggoth - the invasion of the Quaggoth terrain and acquisition of the resources was officially dubbed the War for Below. The War eventually ended, with the Drow claiming most of the Quaggoth territory. 
After being away from the surface so long, the Drow formed their own society in the Below, and are now a well established Stratocracy, who now trade their resources with their once equal High Elves.