Catastrophe

The catastrophe was a tremendously influential event in Seraph of the End, and the one that ruined the world and wiped out most of humanity. That the catastrophe is set to arrive is known during the events of the Catastrophe at Sixteen prequel novel series, and its manga adaption. The Vampire Reign manga and the anime adaption is set after the catastrophe event.

Details
Initial reports detailed the catastrophe as a human-made virus. The most direct effect was the almost instantaneous death of most of the world's adult population. Sudden blood loss from the mouth before collapsing was one indication of the curse effects. Only children were left unscathed overall, though some such as Mirai Kimizuki still experienced effects which involved curse based marks appearing on her.

While there was essentially a total loss of adults, there was partial destruction to the world itself. In particular, infrastructure such as buildings appeared as if they had sustained damage from earthquakes. Oceans became habitats for monsters. On land, creatures such as Horsemen of the Apocalypse roamed the Earth. While the catastrophe was a one-off event, it would change the world further in what beings were able to become active in it.

Background
While attending high school as a student, Guren Ichinose would learn of the incoming catastrophe from second progenitor, Saitō. Although not common knowledge among the population, there were different factions such as the Hyakuya Sect who had intelligence of a supposed preordained event. Rather than take measures to prevent it, there were actions undertaken to survive in what would be a new world, namely the development of Cursed Gear.

Story
An unknown virus is said to have brought the world to an end. Where huge numbers of people are dying, vehicles crash and explosions occur in the cities they are in. Accompanied by a host of vampires and aircraft, third progenitor Krul Tepes reports that humans have created a virus that has spread all over the world. From the catastrophe leaving just children alive, vampires enslaved them to secure a blood supply for themselves.