Great Reality War

The Great Reality War is a major event that happens eons before "Lifestream" takes place.

History
Near the beginning of time, there was three main realities. The lower Reality was named the "Living World", and cannot observe the upper realities. However, it is the only Reality to branch off and create Dimensions out of it's Reality. The two upper Realities was named the "Spiritual World" and the "After World". The purposes of these Realities were simple: the Living World harbored humans or Livings, the Spiritual World harbored Spirituals or Spirits from deceased Livings, and the After World harbored corpses from deceased Livings. By mortal Livings, this unknown cycle was usually called the "AfterLife," but it is only a cycle that keeps the balance over the only three Realities.

However, it seemed that the presences in the After World were envious of the role that the Spiritual World plays, and demanded that they would merge Realities to satisfy the two needs of better reputation for the After World and the faster cycle process for the Spiritual World. Despite their efforts, the Elder Spirits that had control o the Spiritual World Council decided against the demand, as it would mean that regular Spirits would be able to access their corpses and "self-reincarnate" into the Living World, creating a discord to the sacred cycle. The After World was quite enraged by the decision, and in blind response, they declared they will break all ties with the Spiritual World and declare war.

Due to the separation links of the two Realities, the presences in the After World were subjected into becoming Demons, and only were a former shell of what they were before. Now, during this war, they relentlessly attack the Spiritual World left and right. The Spiritual World was able to hold back most of the attacks, but were beginning to fail due to the number of Spirits they had to protect. At the same time, the cycle beginning from the death of a Living stopped in the Living World, making everyone at the time "immortal" until the matter was settled. Seeing that there was no other choice, the Elder Spirits of the Spiritual World Council decided to lend their powers through Spiritual Pacts to Livings that were capable of using them. This created the first usage of Spiritual Pact Bearers, which were quite stronger than the Elder Spirits due to their fully-fledged emotions and other traits that some Elder Spirits lack. Soon, the Spiritual World was able to fend off the last of the attacks coming from the After World. Despite that, a single Elder Spirit decided to dispel themselves from the Spiritual World and ally with the After World, and only Miki was the one to figure out the identity of the betraying Elder Spirit. To her, it turns out that the "Elder Spirit of Shadows" Yasu had done this, and had sympathy for the After World regardless of the consequences for the sake of Reality Balance. From there onwards, the After World was renamed the "Alternate Side" due to the separation links between the Spiritual World and the Alternate Side.

It is hinted in "Lifestream" that Miki decided to make a Spiritual Pact with Kaika Cho because she feared of another "Great Reality War" to occur and that would destroy the absolute balance. She fears this because after all of those years, Yasu could be considered the god of the Alternate Side by this point.

These are known characters to be involved in the "Great Reality War:"
 * Miki
 * Yasu