Era of the Fiends

Summary
The world is taken over by the denizens of the Nether Planes, causing chaos and horrible scars on the lands and its people. The horrible evil of this era caused the world to unite in a way it never had before and end it after only fifteen hundred years.

13,000 P.C. - 12,500 P.C.
By corrupting the most powerful and influential in the Elven society, they demons and devils were able to cause enough infighting and strife so that when they arrive on this world with their armies, the Elven nations fell quickly. The Elves fell within a couple decades, surrendering the entire world to their control.

The Fiends then divided up the world. Demons and Devils do not get along, so they each claimed a continent, mostly staying on their part of the world. Even after centuries of bringing in more forces, neither side had a clear advantage so they remained on their side of the world.

12,500 P.C. - 12,000 P.C.
Various dark endeavors and experiments were made during this time. Several species popped up and died off, although some of these demonic experiments linger on today. Also, various dark rituals performed throughout the world left dark marks on the world.

It seemed, even centuries after they surrendered the world, the Fiends still had a hatred for the Elves and many of their experiments were on the Elves. Because they were targeted like this, their once massive population died out quickly and pretty soon they were a minority on the world. This also led to the creation of the Drow Elves.

12,000 P.C. - 11,500 P.C.
After seemingly destroying their only rivals for world domination, the Fiends didn't anticipate the Humans to make a move. Sick of being enslaved by this race, that power, that race, the Humans finally decided to organize and take charge of their own destiny.

The Humans, resilient and adaptive, used every single weapon at their disposal. They convinced the Dragons and Giants to fight for them. They weren't even afraid to use the Demonic magic against the Fiends. After several increasingly widespread and bloody wars, the Humans finally drove all the Demon and Devil lords back to their home planes...

Signs of the Era
Various creatures and abominations were either created or brought here during this time period. These include various Demons and Devils, Cambions, Drow Elves, Driders, Hellhounds, Gnolls, Jackalwere, Lamias, Merrow, Minotaurs, Nightmares, Rakshasa, Succubus/Incubus, and Yugoloths. Many of them first appeared in this world in this era and some have never left.

Various dark spots remain from this era, locations that were used to perform dark rituals and sacrifices. Many were cleansed and consecrated during the Era of the Gods, but some remain in the depths of the world, in dark places no one dares visits.

There are even some dark and evil magical items and spells that were created or discovered during this era. Some of these dark items and any dark knowledge were destroyed during the Era of the Gods, but a few remain. The power behind this knowledge and these items corrupts most people who wield them and are waiting to be discovered.