The Seven Bloodstains Incident (七つの血痕事件, nanatsu no kekkonjiken?) is the name of the first large disaster that took place in Ar Ciel, which caused the complete destruction of the region of Syestine and served to set in motion the events that would ultimately lead to the world-destroying catastrophe known as the Grathnode Inferia.
During the thousand years that preceded this disaster, the regions of Sol Cluster and Sol Ciel, or better said, their largest countries El Duel and El Elemia were locked in mortal war over religious disputes and the fact that the Clusterians wanted to appropriate the Sol Ciel territories due to the fact that they turned their own land infertile and barren due to their own constant wars.
This conflict was paused several times thanks to armistices and truces, but it reached its most violent and brutal point in 3028 AD when El Elemia tried to put the region of Syestine under their protection, intending to use the territory where it was located to build the third and final Tower of their Ar tonelico Project in order to gain an edge over El Duel on the war, which was a given thanks to Syestine being located in a prime strategic position. El Duel saw this as an aggression and therefore declared war to the Elemians, thus launching the war into its final stage.
Though the Elemians fought hard, by the year 3031 AD El Duel had gained the lead due to the fact that they managed to nullify the greatest weapon El Elemia could use against them: the formidable amplification Tower, Ar tonelico. They did this by deploying a large number of wave-jamming devices called "chaffs" throughout El Elemia's airspace, so any use they tried to give to the Tower would be in vain. Therefore, the Tower's engineers advised firing a stream of Symphonic Power downwards, towards the planet itself, as a way to attack El Duel due to the region of Sol Cluster being located in the exact other side of the planet as them.
However, they committed a grave miscalculation: upon being fired, the Dynamic D-Waves that formed the stream of energy resonated with the internal convection of Ar Ciel's own mantle, greatly amplifying their destructive power. So this shot, which was intended to be a mere demonstration of force that would have caused a middle-level earthquake in Sol Cluster, ended causing a massive eruption of plasma that covered an area of several thousand Stons, large enough to completely evaporate one of Earth's seas.
This failed attack left seven large strangely-shaped craters in the continent where Sol Cluster is located, which are what give this disaster its name, and was directly the cause for the formation of the Great Fang mountain range that exists currently in said region, as the land upheavals the area affected by the disaster were what gave rise to its mountains. It also utterly destroyed four of El Duel's allied countries, one of them being Syestine itself. On the other hand, this caused a devastating damage to El Duel itself, so they lost the top position in the international affairs of Ar Ciel and El Elemia rose to take their place, also putting a definitive end to their millennial conflict.
Likewise, the Third Tower Project in its original form went up in smoke due to the loss of Syestine's territory and returned to being a bunch of blueprints until it was redesigned for a new purpose a few years later.
And the greatest and gravest consequence of this disaster was that the ill-fated warning shot ended cracking Ar Ciel's Heart of the Land and destabilized the planet's own electromagnetic field, which caused several intense earthquakes, severe weather pattern alterations and the complete elimination of the ozone layer. This was why the executives ruling El Elemia formed the AHPP and drafted the Planet Regeneration Project in order to attempt to repair the grave wounds they had dealt to Ar Ciel.
However, the formation of this group would be for nothing, as later on, part of the native inhabitants of Syestine: the Teru Tribe, which were enraged and filled with hatred due to the destruction of their homeland, would deliberately cause an even greater catastrophe a few years later to get revenge on the humans, and at the same time, to exterminate them all.
While Ar nosurge: Ode to an Unborn Star is set in the years after the time when the Seven Bloodstains Incident should have happened, the player can still explore the region of Syestine as it was before it took place, as it seems like the catastrophe hasn't happened yet, despite Shurelia being present and in the process of being turned into Ar tonelico's Administrator, which wouldn't happen until four years after it.
This is one of the greatest inconsistencies fans have found between the Ar nosurge storyline and Ar tonelico's world setting, though it's theorized that Shurelia was included only for marketing reasons and that the time period should actually be prior to the Seven Bloodstains Incident.