The term Fallen Angel came into usage in the Creation War (1486 DE - 8777 DE). 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 to use excessive and immoral measures against the enemy.
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." 
On 3 Lunar 9414 GE, Asmodeus and his lieutenants were found guilty of re-purposing soul energy for their own designs, building secret armies. The ringleader Asmodeus and his lieutenants, all Covenant generals, were stripped of their celestial status, banished as Fallen Angels from the Mortal Systems.
|Asmodeus||3 Lunar 9414 GE|
|Baalzebul||3 Lunar 9414 GE|
|Dispater||3 Lunar 9414 GE|
|Geryon||3 Lunar 9414 GE|
|Mammon||3 Lunar 9414 GE|