The angels were created to act as God's direct instruments in the Symphony. To this end, each was given a Resonance, a particular ability to hear and interpret certain melodies within the Symphony as a whole. The resonance of an angel is more than this, however. It also defines his nature, and to act contrary to his resonance separates an angel to some degree from the Symphony.

Part of the heavenly host chose to do this. Led by Lucifer, they tried to impose their own order on the Symphony. Their revolt failed: Lucifer was defeated by the Archangel Michael, and the demons were driven from Heaven. They found that they were now connected to the Symphony in new ways: instead of perception, their new Resonances were devoted to altering the Symphony according to their own desires.

Then along came humans, and the demons found that they could snatch their souls from Heaven's clutches. The War continues...