Great Gastrea War

The Great Gastrea War is an event that took place ten years prior to the current storyline.

Prologue
A mysterious virus that suddenly appeared before mankind in the year 2021; entering the human body through bodily fluids and causing infections, altering the host's and their form, triggered the beginning of the end. Due to its high contagiousness, it immediately spread throughout the entire globe, easily taking humanity's spot at the top of the food chain. Along with the sudden appearance of the epidemic of the Gastrea Virus, the Cursed Children, girls with red eyes and with the physique of a human, were born into the world—nearly at the same time.

When mothers gave birth to their children during this era, most would immediately capture the color of their eyes, going insane if the color red was present. Additionally, women would sometimes give birth near rivers, to rid themselves of the burden of a Cursed Child by drowning them in the nearby waters. Quickly after its breakout, the entire world went into war against the deadly parasites.

War
Humanity rages war on their greatest enemy yet, Gastrea. To confront the great enemy, armed men are sent into the battlefield. Supporting them were their respective commanders, reciting ample words to keep them fighting; reminding them that they are fighting to protect their family and friends, and that if they fail, their loved ones will all die at the hands of the alien creatures. With the end of the world resembling the current scenery, fighter aircraft fall from the skies, men collapse at the hands of the enemy and all hope is lost. Somewhere along the line, Mechanical Soldiers were created to fight against the Gastrea, titled as the "New Human Creation Plan." Albeit, when the abilities of the Cursed Children were discovered, the creation was put to a stop. Soon, Japan was heavily invaded by Gastrea. Shortly thereafter, the country declares its lost, quickly joined by several other countries. Eventually, mankind declares its lost in the war against Gastrea. Furthermore, the Ladder of Heaven, a weapon stipulated as the strongest and evilest, was born from the Great Gastrea War, bearing witness to humanity's defeat.

Aftermath
After losing the war, Japan and many countries barricaded themselves within Monoliths to counter the Gastrea. In the following ten years, humanity was finally able to rise their impoverished living standards back to that of the first half of the 2020 decade. However, once in a while, Gastrea would sneak in past the Monoliths. In order to fight against them, the humans organized two-man combat teams to specifically fight against the Gastrea: a Promoter and Initiator. To guarantee the safety of detectives, a new law is created, dictating that in any incident where Gastrea is involved, the detective cannot enter the premises without the accompaniment of Civil Security.