During the calmest day people can think in Zaraki, several small Garganta ripped open and Hollows came out attacking the district. Some non-humanoid Arrancars that are below Espada level, joined attacking Zaraki and Kusajishi. They attacked the people, instead of devouring them and destroyed their houses, without letting Shinigami sense their presence. The invasion lasted for half a day, with the district completely devastated.
Only a few of the citizens that resided in Zaraki survived through escaping from the village, but some were cornered by Hollows and killed. Asuka Aikawa was a native in Zaraki and was one of the Souls that tried to escape, but was stuck in a group of Hollows. Even so, he remembered the phrase his sword shouted a day before the invasion and screamed it out. Shadow Reiatsu emerged from the sword, bursting the Hollows that surrounded him. Michiko Kōno, a villager in Kusajishi, managed to pass through a horde of Menos Grande by focusing her element type reiatsu, fire, in her “silent” Zanpakutō’s blade and attacked each mask she sees. She wore a cloak that made her presence hidden from the Hollows as she wandered in a very long forest that lead to Inuzuri. She arrived in a silent village she thought was West Rukongai that was very relaxed and didn’t know what happened.
The Arrancars and Hollows attack Inuzuri next, in just a few hours after Michiko arrived in the district. The invasion was not yet grave until it spread around the West and East Rukongai. In Sabitsura, the 64th District of East Rukongai, the Hollows were already there when Haruo Yamada and Mitsuru reunite and went back to Inuzuri where the Yamada Manor was placed which was slightly damaged. They then fought nearby Hollows to protect their house.
The Hollow Invasion stopped. Seireitei was also half-destroyed. It was when Hisa of the Yamada Family died for an unknown reason. Her death caused shock and fear to the family.
On the same year shock and fear was inflicted on the Yamada Family, the Hollow Invasion reborn. Their house was destroyed by strong Hollows and nearly all of the people living in there died. Some members managed to survive because they hid their presence.
In the same district but on a different year, a member of the Fujimoto Family, Yuri, was taken to the Hueco Mundo by an Arrancar. The residence of the Fujimoto Family was left with major damages and Yusuke Fujimoto and Yuuji who were still young. It was too late when a group of Shinigami came, the only help they can give is healing them.
Years later, in a forest in Inuzuri, Izumi Yukari and Runa Kiramekiwere chased by a Hollow and later saved by Yusuke, currently a Shinigami. Probably on the same year and in the same district, Mitsuru, Haruo and Michiko were attacked by an Adjuchas and were fatally injured. Curiously, Mitsuru and Haruo’s bodies were found in the place where their mother died. Later at evening, Michiko woke up in a forest and went back to her family without any memory about the incident.
In other districts in Rukongai, the Hollow Invasions was thin and mild unlike in South and North Rukongai. Seireitei was also majorly damaged, but the Gotei 13 managed to undo them before the final years.
In the Hollow Invasion era’s final years, many Shinigami helped ended the Hollows’ lives, but they just continued multiplying. It ended about 3-10 years ago when an unknown Shinigami ended their multiplying ability. Further information about the invasion remained unknown until now.
It is known that the Hollow Invasion era is just an arc during the past which had probably inflicted different damage to different people today and in the future as well, as there are more people to join as far as we know.
Most of the characters’ pasts were during the mild Hollow Invasion era where several Hollows were still invading the Soul Society, due to the fact that it first occurred approximately 110 years ago and just ended 3-10 years ago and the most recurring people in this invasion are Haruo, Mitsuru, and their family, and Izumi and Runa. This is because the arc was made for their past and since that there are some characters that had trouble with Hollows before, it became the Hollow Invasion era.
The most possible origin for this is that when two masked people were standing on air before the invasion, they must had broke a tube very similar to a Hollow bait, that even attracted Adjuchas. Some Arrancars must have joined, too, for Souls. They are probably Quincies that wanted revenge on Soul Society.
It is based on a Horror Story that was actually an apocalypse-like, but it was just made an invasion.
The invasion also happened in Seireitei, but they only managed to end theirs before the final years. This can be a possible reason they weren’t able to send stronger Shinigami to help in Rukongai. Note: There were fewer high-ranking Shinigami before.
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