Sol Cluster (ソル・クラスタ, soru kurasuta?) is the region that serves as the setting for Ar tonelico Qoga: Knell of Ar Ciel. It also used to contain another smaller region called Justine, but it was destroyed during the Seven Bloodstains Incident that also served as the conclusion to the thousand years war they had with the region of Sol Ciel. This also caused the formation of the mountainous region we currently know by the name of Great Fang.
Its former capital used to be the city of El Duel, whose leaders aggressively attacked and annexed all of the other cities, countries and kingdoms that were located in this same continent, forming an alliance of 48 countries united haphazardly under a single government. After its fall, the region was governed briefly by the Kurogane Laboratories, until Clustania was founded and after a certain incident, they personally dismantled the Laboratories. Later on, the Archia Think Tank was founded by the researchers that were once aprt of the Kurogane Laboratories, which have competed with Clustania for supremacy ever since its foundation, although it also had a hidden agenda of its own... The other main area of this region are the mountains collectively known as the Great Fang, which are dotted with a few settlements, villages and cities populated by humans, although the Reyvateils from Clustania will come and personally destroy any of these settlements when they have grown too much, or if they actively rebel against their government, in a process known as Cleansing.
Historically, it could be said that this region was the cradle of human civilization in Ar Ciel, as this is the place where the earliest records of civilization can be found, and where the first kingdom was founded back in the 112 AD year, which was composed of 22 countries and countless settlements. Later on, the monarchy launched an aggressive steel production plan which consisted of invading and capturing other settlements to force them to mine and refine steel. Much later, all of that was unified into the great empire of El Duel under the rule of Gurentei (Yurisica), although this also caused the formation of the Western Cluster Alliance, which rebelled against El Duel and initiated a hundred year-long war that devastated most of the land and left it unable to produce any food. This prompted the inhabitants of Sol Cluster to begin exploring beyond the ocean in search of more land where they could settle in 2020 AD, and it was through this that they finally reached the region of Sol Ciel and established contact with their inhabitants. Initially, the relationships were rather good and even managed to export some of their technology to Sol Ciel, particularly the smelting the Cielans began using to make music boxes. However, due to their cultural and religious differences, particularly the Harvestasha legends from Sol Ciel and the Liarsha & Syerek legends from Sol Cluster, and due to the fact that the Clusterians were so power-hungry, both sides began a war that actively lasted for around 700 years, until an armistice was signed in 2775 AD. However, they were strictly speaking locked in a cold war, which only escalated with the passing of time until it finally exploded into the two catastrophes that destroyed the Planet's Heart of the Land and rendered its surface unfit for life: the Seven Bloodstains Incident and the Grathnode Inferia. These last two facts were what forced Sol Ciel to send over a team of their own researchers and one of their Reyvateil Origins, Tyria, to create the Third Tower of Ar tonelico in an attempt to regenerate the Planet, but due to several problems, this was impossible to realize and the Tower was left incomplete until the current days. Likewise, said Tower ended being perceived by the Planet as a threat due to its Moocheriel facility, and this is why it began producing the Antibodies, making Sol Cluster the only region in the world where said creatures frequently appear.
Prior to the great catastrophe, it used to have an humid and harsh climate, and while less harsher than Metafalss due to being located at a much lower altitude, its inhabitants had to be very diligent if they didn't want to die from starvation. However, they also were very ambitious and wouldn't stop at anything to accomplish their goals, even if this meant robbing other people and regions from their resources.
Geographically, Sol Cluster contained the mountain and forest belt of Justine, the Mt. Dauye volcano, and it also used to contain the countries of El Duel, El Nemesys and El Daijyella; of which only El Duel continued until the end of the First Era, as the others were either destroyed or absorbed by it. Sol Cluster is located exactly on the opposite side of Ar Ciel from where Sol Ciel is, which was why the Cielans tried using against them the attack method that unintentionally caused the Seven Bloodstains Incident.