Primordial War

The strife generated by the Primordial War rocked Creation, destroying many things that would never be made again. The five great Elementals were unmade by the Primordials to prevent them from being used as weapons by the Exalted. Also, the pre-human race called the Dragon Kings died by the millions - those who did not were left with a choice of continuing to fight (which could lead to the death of their race) or to retreat into torpor and hope for a revival in a future age.
After years of battle, the Primordials were defeated. Some were slain, which caused a horrific Abyss to open beneath them and swallow their dying souls, but others surrendered out of fear of the very same Abyss. These were spared, at the insistence of Gaia, the only Primordial apart from Autochthon to have sided with the Exalted. Those Primordials spared death were forced to swear oaths on their names that banished them to Elsewhere and imprisoned them within the body of their general, Malfeas.
The Primordials who died became known as the Neverborn. Too vast and powerful to pass into nothingness (or Oblivion) but also not governed by the rules of reincarnation that had been created for mortals, they lingered on at the doorstep of non-existence. Also, their collapse into the Abyss damaged the very cycle of reincarnation and the Essence that made up Creation. This pollution came together in the form of a parallel world that has earned it the moniker the "Underworld".
The Primordials who were imprisoned became known as the Yozis, or Demons. Almost as soon as they had been imprisoned by the Gods, these beings began to seek escape and a means of revenge upon their upstart creations. Within their prison they created races of offspring to serve and entertain them.
However, their defeat was not total. Even in death, the Neverborn cursed the Gods, the Exalted and Creation as a whole.