The Civilization of Ashes was a great state of mortals which grew under the tutelage of the Ten into an empire that rivaled the Fallen in power and insight. It ultimately fell, however, because humankind was not ready for the secrets they had learned.
After the Time of Atrocities, Lucifer devised a new strategy in the war with Heaven. The angels would teach humans the secrets not just of Creation but of Heaven as well, raising them up to god-like levels of power and insight, and thus enabling them to cast the Host out of the world themselves. Then the rebellious angels would be able to conquer Heaven and replace God and God's loyalists.
To this end, Lucifer commissioned a group of his followers called The Ten to go forth as tutors and advisors to humankind. The Ten taught humans as they were commanded and found them eager students. While the Ten could not be everywhere at once, they left behind textbooks such as the Canon of the Eye from which humans could study on their own. For more than a hundred years, humans learned tremendous knowledge and grew in power and potential. Angels called this era the Time of Babel.
At the height of the Civilization of Ashes, jealous angels unleashed the Nephilim--half-angelic, half-mortal monsters born from the rape of human women--who conquered the cities and killed the Ten. The Nephilim ruled the Civilization of Ashes for a brief time, until Lucifer brought the Legions and personally struck them down.
Moreso than this war, however, the Civilization of Ashes crumbled because humankind was not yet prepared for the knowledge they had been given. They turned their backs on their grand cities, and on the angels who had mentored and protected them, and even the First Tongue that united them. They splintered into numerous different tribes speaking different languages, an event known as the Shattering.