Holy War

The Holy War is an event mentioned in the story, Highschool DxD: Til Midnight. It was mentioned by the Elder God, Amatsu-Mikaboshi, during a meeting of the Celestial Council.

Overview
The Holy War was stated to be a cataclysmic war between the Forces of Heaven and the Forces of Hell. The Elder Gods sided with the forces of Heaven and assisted them in defeating the demons driving them back to Hell.

Outcome
The demons were driven back to Hell by the Elder Gods. For reasons unknown, most of the Archangels except for Azrael lost a majority of their powers causing them to be slightly more powerful than the Ten Seraphim.

Trivia

 * The Holy War is based on an event of the same name in the series, Nanatsu no Taizai.
 * It is often referred to as the First Apocalypse
 * Many of the Elder Gods of today's religions are survivors of the Holy War with an unknown number of Deity Class beings that were eliminated during the War itself.
 * Azrael is the only Archangel not to lose a majority of her power during the Holy War. She only lost a small amount.