The Third Age

The third age kicked off unexpectidly when demons poured in through a rift believed to have been dug into by dwarves. Many civilizations fell under the brutal attacks and in despiration hire powers were beseached. Devils came rushing in under the banner of Tiamat to do battle with the demon destroyers but this helped weaken the barrier between the plains. Elementals in their parrelel plain took footholds of their own. Finally the celestials came, not meerly to defend the people of the world, but help drive the demons back to the Abyss and seal up the rifts they had torn. Even with that combined force it wasn't enough. Only the Triumbra, 3 god like beings working together, were able to finish what the others had started. The Electric-Shadow Basalisc, the Shadow-Fire Nightmare, and the Fire-Light Phoenix.
The world was never the same. Weak points between plains still exist in areas, such as the Forbidden Ocean. Demons, Angeles, Devils, and Elementals all 'touched' the mortals brining forth new races. But the people were convinced that digging into the underground would lead to more demons or something worse... and the conquest of trapped minerals and metals was abandoned by all known civilizations in a way that has held even till today.