A few thousand years ago the demons enacted a ritual of planar desecration and all of the planes of reality collapsed into one reality. Only the elemental planes remained apart, but large portions of them were drawn into the cataclysmic ritual. What they thought would result in the triumph of their home world, the Barrens, became the most mismanaged and failed attempt at eldritch magic. The demons were unable to control the elemental storms that ravaged the lands and rearranged the entire world of Arenas. However, they were able to slay all of the titans, and all of the gods. The locations of their death are some of the most ravaged and haunted places in the world. The Krovs (Lords of the Undead) were slain, but not before they unleashed powerful necromantic magic that filled the land with undead and deprived the demons of the souls needed to rebuild their armies. The demons then turned on potential allies, the drow and the githyanki, and nearly exterminated them in an alliance with mind flayers and other denizens of the underdark. By the end of their invasion their own numbers were greatly reduced and much of the world lay barren and desolate. The sun and stars were banished from the sky, all that remained was the moon, its blood red color a grim reminder of this age of endless slaughter.
Eons have passed since that era of war and extermination. The demons persist, but are far from unstoppable. Death is a reality for all and even now an immortal demon can be banished to oblivion if they fall in battle. Planar travel is nonexistent, it is unknown if other planes of existence persist. Rumors stir of a new god that offers vengeance and seeks to drive the demons from this world. Some say he is crazy and prone to fits of intense violence and fails to distinguish between friend or foe. Gnomes have emerged from exile beneath the world and bring with them new knowledge of elemental tech. In the brief century of their return the world has become dominated by the use of airships, locomotives and new weapons. All that remains of the world before the fall are legends. There remains little hope for the living. However there are cities that have declared their freedom and such places might offer hope to the adventuresome. Rumors persist of a way to reverse the damage done to the world. If such a thing is possible, it is a tightly guarded secret.