The term Fallen Angel came into usage in the Creation War. It was name given to those that had lost their way, becoming more like those the Nawirrûs Covenant were battling. The first to earn this inglorious title was Dališar.
In the God Era, a new Covenant, similar to the last, battled the demon hordes spilling out of the Abyss. This conflict, the Demon Spawn War (8972 GE - 9493 GE), was so long and brutal that it drove some angels to use excessive and immoral measures against the enemy.
We must fight them on the same terms, our methods must be as brutal as theirs if we are to stop the hordes, and if our agents are to operate among them, they must act as them.
In 9414 GE, Asmodeus and his lieutenants were found guilty of re-purposing soul energy for their own designs, building secret armies designed to combat the demons and weather the hazards of the Abyss. The term "devils" came to be used for the creatures gathering across Ellubôz's forge worlds. The ringleader Asmodeus and his lieutenants were stripped of their angelic status, cast out of the Mortal Systems. They were sent to the worlds they had stained with conflict where the brutality of the Demon Spawn War was taken to the extreme.
Asmodeus smiled, and the smoke of a thousand battlefields rose from his lips. "As Lord of Battle", he pointed out, "you should know better than any that war is a dirty business. We have blackened ourselves so that you can remain golden. We have upheld the laws, not broken them. Therefore, you may not cast us out." 
- Asmodeus, to Heironeous - "Trial of Asmodeus"
|Asmodeus||Overlord of the Hells|
|Baalzebul||Lord of the Seventh|
|Dispater||Lord of the Second|
|Geryon||former Lord of the Fifth|