Blue Night

The Blue Night was an event that happened sixteen years ago, during which Satan killed numerous powerful holy men and Exorcists from around the world by attempting to possess their bodies.

Manga
During Lewin Light and Ryuji Suguro's investigation of the Blue Night, Mephisto Pheles explained the origin of how he and his brothers and sisters came to be.

In ancient times, they were considered conceptual beings known as Ba’als that did not possess a physical form and were situated far away from humans without the possibility of physical contact. However, as human civilization developed, humans began to worship and call out to the Ba’als, granting them distinct forms to the point of assuming flesh and blood.

However, while Ba’als were immortal, conceptual beings, the humans that they possessed were not and withered as time passed. Having to go through a physical death with every vessel, Ba’als still continued to exist as conceptual beings after death because humans continued to believe in them.

From having to suffer constant death and rebirth, Lucifer had built a strong hatred for humans and eventually turned his frustration towards the humans, invoking wars, floods, and epidemics. One night at a dinner gathering among his siblings, Lucifer attempted to wipe out all of existence to end the constant struggle. Mephisto convinced him to stop, suggesting that humans were also key to their physical existence. His solution was to attempt creating an endless supply of bodies for Lucifer to inhabit.

The solution brought about the creation of Section 13 (under the guise of "Asylum"), a secret underground facility used to experiment on humans to form vessels. Experimentation began with cloning of vessels, which led to many problems as clones were not exact reproductions. This resulted in a switch in perspective, creating the need for an elixir that replicated the effects of a Ba’al's rapid cell regeneration. The genes from the top three Baals were used to form the elixir, which they then fully submerged human bodies into for creating the reinforced humans.

As successful cases began to emerge and the process became routine, one such reinforced vessel that was deemed the "strongest reinforced human to date", harboured what was believed to be the realm of Gehenna itself, which allowed for Satan to enter the human world and the events of the Blue Night to transpire.

Anime
The true motives behind this night are addressed in the anime as Satan's attempt to rescue Yuri Egin from being burned at the stake for carrying Satans child. Possessing various people in an attempt to find a host who could survive being possessed by him long enough to save Yuri, he eventually took control of Ernst Frederik Egin, freeing Yuri before his body gave out.

Repercussions
The Blue Night caused massive upheaval in the Exorcist community, among others, becoming common knowledge among those affiliated with demons and organizations who deal with them. Just some of these consequences are listed below.

Organisation Changes
Following the Blue Night, multiple exorcists associated with Section 13 were reassigned and a branch wide cover up was executed to hide the reason for it with all the survivors taking a Morinath Contract to ensure secrecy.

The Myōō Dharani sect, after having many of its monks and priests die simultaneously, became known as the cursed temple, leading many of their followers to flee and abandon the sect. This forced them to align themselves with the True Cross Order in order for their sect to survive. Other sects saw a similar exodus in the wake of the Blue Night, though details of the effects on these groups are scarce.

Injuries
Despite the fatal nature of being possessed by Satan, not all victims of the Blue Night died, nor were all those possessed mortally wounded.

Deaths
Of those killed during the Blue Night, not all were a direct consequence of possession.