The War In Heaven.
Also referred to as the Battle In Heaven.
In the time before time, there was a battle in heaven in which one third of the angels in Heaven rebelled against God. The rebel angels were led by Lucifer, formerly known as the brightest and greatest of the angels. These forces were defeated by Michael, who is said to remain the greatest of the Archangels. These stories are originally told in the Book of Daniel, and can also be found mentioned in the Book of Revelation and the Dead Sea Scrolls. For a summary of these discussions, see the Wiki article on the subject of the War In Heaven.

"Hell's Reign to Serve In Heaven", by Travis AKA DigitalBaptism, featured with permission. For more of his creative work, please visit him at DigitalBaptism at DeviantArt.
The stories passed on to us by our ancestors do not delve into a great amount of detail on this epic conflict. We, in Hell, can tell you that the reports that have filtered down through history are as a rule and a necessity the story as told by the winning side.
This war was not short, nor clean, nor purely "black and white", good versus evil, as we would like to believe. It in fact lasted for ages, for amounts of time beyond the mortals' capacity to comprehend. This conflict consisted of many battles fought over unfathomable spaces in the universe, and for amounts of time that dwarf all of the known recorded history of Mankind. This war is a tale of courage, tragedy, heroism, and villainy.
The portrayals by many classical artists seem to concentrate most on a moment where the Archangel Michael defeats Lucifer, or slays the Dragon or hurls it into Hell. But this story is so much larger than that.
To learn more about the War In Heaven:
Paradise Lost: Angel Raphael recollects to Adam
Paradise Lost video.
War In Heaven (by Bill Ratekin)
War In Heaven
What are the details of the War In Heaven?
Heaven.net.
Angelology and information on the War.
Steliart.com.
"Archangel Michael", by Guido Reni (1575-1642). Circa 1636.
Illustration from "Paradise Lost, by Gustav Dore. Circa 1866.
"Fall of the Rebel Angels", by Pieter Bruegel the Elder. Circa 1562.
"La Chute des anges rebelles", by Chalres Le Brun (1619-1690). Circa 1680.
"The Great Battle", by Kaana Moonshadow, featured with permission. For more of her delicious work, please visit her at Kaana MoonShadow at DeviantArt.
Painting at the Saint Michael Parish Church at Untergriesbach, by Johann Georg Unruhe. Circa 1793.