Known as the Hraen to elvenkind. The Dark Below is a subterranean supercontinent of infinite chambers, galleries and rivers formed as a result of the breaking of the world when the gargantuan, star-dwelling manawurm Naug crashed into nascent Nhera. The wurm sought to feast on the condensed aether of the The Starwell.
Naug was so massive that it tore the elements of Nhera apart, creating chasms wider than even divinities could see, requiring the creation of the first seas by the Eotenari to assist the elves with a means to traverse the now-broken world. The first expeditions to leave Valurn were in an effort to find lost kin that were separated from them. Most of those that fell into the Hraen were not found until some ages later, having become what are known as Druhau.
Naug was also responsible for the seeding of the world with aberrations from outside the veil that clung to its hide and were deposited upon the Realmstar. Most of these creatures and things ended up within the dark below, where they have remained ever since as there are few easy ways back to the surface.