Divine-Demon War

The Divine-Demon war was a conflict in the distant past.

History
At the beginning there was a god who created the world and to manage it he gave power to eight personns called the transcendentals. Each of them had a special power called "authority" allowing them of incredible deeds : Lina had the power to foresee the future and Ruyo the power to destroy anything he judges a threat to the world (Annihilate). To put limit to the transcendants one, the god established the rules (preventing the future wise gods to intervene endlessly and recklessly in the world). At a unknown date 6 great high spirits appeared and became the protectors of the non-human races (the spirit folk).

One day the god left the world and six of the transcendantal beings became desperate. They searched a way to know how making their god come back. Lina refused to help them and got restrained by the six other gods. The late began to experiment on space-time magic and opened a gate to a infernal world and summoned the demons on the world. Panicked, they free Lina and begged them to help them to correct their mistake. Lina accepted but deep down she lost her trust in her fellows companions. The plan was established : she would look for the high-spirit's and Ruyo 's help, meanwhile the others would keep the demons at bay. The future six wise gods lead the humans in the fight, granting them magic (which would become the modern magic through contracts with formula).

Lina went to the spirit village and asked the high spirits and spirit folks for help against the demons and they accepted. While they join the war, Lina looked for Ruyo, the most powerful transcendantal, in the future Yagumo regions, he also accepted to join the fray after freeing Sora, his sole familiar. After their return in the Strahl regions, Lina found out her fellows had betrayed their allies and her. The six transcendantals had been imprisonned the six high spirits to create the hero system (the spirits are in the divine arms of the heroes). It is unknown they did this out of malice or because the state of war required it. Nethertheless Lina and Ruyo succeed to put and end to the demon war.

After the final fight, Ruyo was dying, probably due to overuse of his authority. Lina had a vision : a thousand year later a great disaster will come and Ruyo will be needed. She decided to reincarnate Ruyo and herself, she divided her divine part to create a new spirit and attached it to Ruyo's soul (to guide him in the future) while her human part will reincarnate into a human girl.

Aftermath
The divine war concluded with the human's victory but a great cost :

The seven wise gods disappeared from the world. Any traces of Lina was erased and the church of six wise gods was established. Despite their help in the conflict, the spirit folks were considered badly by the humans and they isolated themself in the wild lands preventing the spirit arts to spread in the Strahl regions. Now locked-up in divine weapon, the six hight spirits are in perpetual hatred for Lina, believing she betrayed them. Soon after the human civilization flourished and they create kingdom.

Sora feels her connection with Ruyo be cutted off. She secluded in the mountains but not before spreading the legend of her master who saved the Yagumo regions before heading to the Strahl regions to fight in the divine war.

Trivia
Despite being a mystery, the divine war is the main trigger of the current story: the reincarnation of Haruto, the summoning of the heroes in the Strahl regions, and even Reiss is some sort tied to the divine war.