The Time of Babel was the last great experiment by Lucifer during the Age of Wrath. Rather than simply protect humans from the Heavenly Host, Lucifer wanted to teach them Heaven's secrets and uplift them to be God's equals. To do so, he commissioned ten rebel angels to tutor and guide mortals into building a vast empire known as the Civilization of Ashes.
The rapid growth of the Civilization angered some of Lucifer's allies, particularly Abbadon and Asmodeus, who resented the thought of humans ever equaling or surpassing angels in power. They arranged to undermine Lucifer's experiment by raping human women, resulting in the birth of the Nephilim. These half-angelic monsters rapidly conquered the Civilization of Ashes and murdered the Ten, and after Lucifer had disciplined their creators he lead a war to cast them down.
However, the Civilization of Ashes was always doomed to fail even without this interference. Humankind was not ready for the divine secrets the Ten had imparted on them, and in an event called the Shattering, they turned their backs on both their great cities and their angelic tutors. Humans also splintered into numerous tribes, each of which spoke its own language instead of the First Tongue they had been taught. The original mission of the Fallen had finally failed, and not long after the Shattering they were finally defeated by the Hosts and imprisoned in the Abyss.