Border Disease

The Border Disease 極限病 (kyokugen'in) is a severe condition that only happens to humans that have been exposed to a significant emotional shock, or witnessed an extremely traumatic event. It has only been observed on the inhabitants of Sol Cluster.

This disease basically manifests as a loss of control of the person suffering it, along a dramatical increase in physical strength and resiliency. A few people can manage to overcome the symptoms and live with the disease, while benefiting from their enhanced capabilities. However, most of the patients end losing completely their sanity and humanity, and turn simply into empty shells who only live to kill and hunt other humans. These empty shells are known as Ultimate Border Humans, and are treated exactly the same as if they were monsters. Treating or curing these people is impossible at this stage, so the only solution for dealing with them is killing them.

The Border Disease has its origins on the Human Evolution Project that the Neo Atlas Faction of the Archia Think Tank developed to attempt to acclimate to the dying planet of Ar Ciel. This consisted in fusing the humans with Antibodies, using the Reyvateils as adhesive between both life forms. To do this, they first modified the Reyvateil genes, to make them active regardless of who was carrying them and then, implanted them into test subjects. Then, said subjects passed them down to their descendants, eventually spreading to most of the inhabitants of Sol Cluster.

These genes were supposedly for allowing the people to awaken as Reyvateils, and allow them to gain the ability to sing Song Magic. However, this was an utter lie, since these people, better known as Medleys), are unable to connect to an SH Server in any way, but the active nature of these modified genes raises their H-Wave levels, which is in itself the real cause of the disease: when one person receives a massive emotional shock, his or her H-Wave levels, already at an incredibly high level, are pushed even farther due to the modified genes. On most cases, the mind of the Medley is unable to bear such levels of energy, and collapses under the extreme strain, turning that person into a Ultimate Border Human.

Aoto also suffered this disease during the events in Ar tonelico Qoga: Knell of Ar Ciel, and he would have turned into an Ultimate Border Human if Saki had not offered herself to share her Soulspace with him, which ultimately ended saving Aoto's life. Hikari Gojo didn't approve of the operation that was needed to give half of Saki's Soulspace to Aoto, since the researchers of the Archia Think Tank were the ones who performed it, but he also said that they didn't have any other choice.