Eclipse is a mysterious occurrence and a phenomena in the real world as the Spirit World collides to it due to concentrated flow of the Spirit Veins mainly attracted to the human activity, emotions, ambitions, etc.
Its basis of appearance started at the end of the Great War or the Second World War. Some however were considered to have happened during the medieval era including legends and other occult stories like the Durham Mystery. Since then, the flow of the Spirit Veins became more active because of unknown mysteries accounted. Due to the rapid activity of the spirit vein, the Eclipse represents its presence by a certain "Gate" that enables anyone and anything to enter or be forced. These gates however are connected into certain places of the Eclipse named by the Underground as Labyrinth or Xanadu. Each are also guarded by powerful beings and minions mainly called as Greed. These beings also follows a hierarchical positions like the Elder Greed, Grim Greed, etc. The knowledge about the Eclipse and its entirety is only known by the people of the "Underground" whom have sworn to secure and use their technology to ensure its stability. The "Underground" with its two main organizations "Nemesis" and "Zodiac" are known to have been the best, having established more branches around the world. Yet, because information is not impossible to stretch, minor groups have gathered to help while some use the Eclipse for their advantage.
The Eclipse, being a parallel world to the reality, has its tendency of to bleed throughout the planet. At first, no one knows how to predict its occurrence until the organization "Nemesis" developed a prediction program called "Horus". The program's basic operation however is to gather information inside the world wide web such as blogs and reddit-like sites especially to "NiAR" and constructs the predictions into location and percentage of Eclipse Occurence. Due to its gathering prowess as the state of the art technology, the "Underground"'s efficiency in dealing with its appearance resulted into better countermeasures. Still, its predictions were not created to stop the creation of the gates and instead to locate its area so that specialized personnel(s) will deal with it inside.
Several incidents and calamities have already been logged due to the Eclipse's occurrence. However, the game only featured incidents inside Morimiya City in the year 2015 and mentioning the first Eclipse Calamity called Tokyo Twilight Disaster.
Tokyo Twilight Disaster
The Tokyo Twilight Disaster is the first ever known calamity brought by the appearance of the Eclipse. It occurred on March 15, 2005 at 11:38 am with normal weather activity when a sudden 7.5 magnitude earthquake hit Tokyo. This triggered the change of sky's color from blue to twilight, engulfing other places into flames, leakage of hazardous gases, as well as thunderstorms, tornadoes, tsunamis, and snowstorms. The unusual weather activity was naturally stated into reports however the "Underground" states it's caused by the Spiritual Type Greeds which are "Kins" of the Mythological Class Grim Greed.
According to Asuna Hiiragi as she knew her parent's work back then, the Eclipse bled faster into Tokyo with its epicenter of the "Gate" is the city itself. It also caused the appearance of unnumbered amount of Greeds including the Elder Greeds and Grim Greeds.
At that time, Sousuke Kokonoe was also involved with the assault alongside Seijuro Hokuto and other unmentioned allies. Their group also stumbled and met Rem in the process and their group was able to defeat the greed behind the Twilight Disaster.
In its aftermath, the lives of several main characters of the series was changed. This includes Kou Tokisaka who was with Shiori Kurashiki at the start of the disaster. Shiori was killed by a falling debris after she pushed Kou to safety which was supposed to hit him instead. Shiori in turn became the vessel of the Mythological Greed after Kou wished for her to be revived. Asuna's parents was killed by the Greeds after attempting to protect her and other civilians, later on she became an Enforcer of Nemesis like her mother and inherited her Soul Device. Rion Kugayama was severely injured from the disaster and had no chance for to live. Until she got contact with a Grim Greed and saved her by becoming its human vessel. This made her an Angelic Possessed for 10 years.