Legend of the Void Rod
In the beginning of history, Luxaz, after being expelled from the heavens, created a terrible, magical item. The item became known as the Void Rod. The evil god used the Void Rod to taint The Great Father's creations. He hated The Great Father and he wanted to steel His authority and power. Luxaz formed the Void Rod in a volcano out of Steel and Brillinium, which created a compound known as Flamesteel. As it was forged, he infused it with magic. Once the Void Rod had been created, Luxaz started corrupting the light species (humans, elves, dwarves, gnomes, dragons, etc.) and transforming them into the dark species (e.g. orcs, goblins, dragonborn, drow, etc.).
Fifteen hundred years after the eight gods and goddesses came to Octisafell, the dark species revolted against Luxaz and his abuse of them. They rose up, stormed his stronghold, and captured the Void Rod. They then fled underground to the Black Beneath and formed a Subterranean Civilization. The people of this civilization slowly came in contact with other denizens of the underground, such as the dwarves. This is how the legend came to be and began to spread to the inhabitants above ground.