The Undouka Wars is the term used to describe the series of conflicts between the Gotei 13 and the Inboyuigon. The spark of the Undouka Wars officially began during the first full-scale battle between the two forces, centered on the earliest conflict of the war, which revolved around the Inboyuigon's infamous theft of the Zanpakutō, the Sōkyoku. Currently, the Undouka Wars in general refers to the war between the Shinigami and the Undouka. Chronologically, the Undouka Wars have been fought by both sides for over 10,000 years, and is currently the longest standing war fought by the Gotei 13.
Some several decades after the Spirit King's royal guard discovered the Vale of the Inochigami, only a small fraction of the guard became enthralled in discovering it's secrets. After the lure of powerful arts began seducing the more curious members of the guard, the Spirit King's royal council saw a potential threat, and exiled the members of the royal guard who refused to abandon the pursuit of the "angels". By that time, however, approximately one third of the royal guard were devoted to the temptation of hidden powers. Exiled from the realm of the Spirit King, and banished from most of Soul Society, centuries later, the deviants discovered a way to enter Kongoukai, and resettled in the pocket dimension for a nearly a millennium. Most in the Spirit King's council believed that with their banishment, it was the end of the obsessions with the Valley of the Angels. The though of an eventual reemergence of the banished Shinigami was inconceivable. Unknown to residents of the Spirit King's realm, was that the exiled Shinigami discovered a way to travel back to the Spirit King's realm, and continue uncovering the secrets within the ruins in the Vale of the Inochigami.
First Seireitei Invasion
The emergence of the Inboyuigon came some two thousand years after their banishment. With the aid of the Spellcasters, the Undouka were able to merge the dimensional planes between Kongoukai and Soul Society, with the focal point being the center of the Seireitei. With a fighting force of over 5,000 Undouka warriors, including ten Guardians and three Councilors, the Inboyuigon launched a major assault on the Gotei 13, both revealing their existence, and making the first strike in the war that followed their move. The First Seireitei Invasion was the first major battle between the Undouka and the Shinigami.
With the element of surprise, the chaos instilled in the unprepared Shinigami, and the use of exotic spells, weapons, and techniques, the Undouka rained down from a merged point in the sky. With the Shinigami panicked against their enemies, the Undouka had effectively broken the back of the Gotei 13, killing several captains and lieutenants, and holding their own against the additional corps. The first contact between the Undouka and the Shinigami marked what many believed to be the true beginning of the Phoenix War.
In the ensuing battle, the Undouka Guardians made their way towards Sōkyoku Hill. It is unknown exactly what the reason was for the Undouka's desire for the Sōkyoku, though it was alleged to that the Shamans regarded the Sōkyoku even more special than the Shinigami, believing that the Sōkyoku's power was not just that of destruction, but also rebirth. By the time the Sōkyoku was secured, the Undouka forces left the Gotei 13 crippled and devastated, and despite the death of several Captains, they would quickly regain their strength.
Battle of Vazaku
The initial Shinigami retaliation to the Undouka came some four hundred years after the First Seireitei Invasion. By this point, the influence of the Undouka had become so prevalent, even the council of the Spirit King had ordered the Central 46 Chambers to take action against them. In light of their previous defeat, the Gotei 13 planned a trap for the Inboyuigon. With the aid of information provided by the Spirit King's council, the Gotei 13 discovered a second ruin in the 82nd East Rukongai District, Vazaku, and discretely spread information of it being guarded by the Shinigami of the Gotei 13, and bearing symbols of the Kikōō. True to their desire of information of the Angels, the Inboyuigon took the bait and dispatched a significantly sized battalion of Undouka to the location to launch a quick strike on the Shinigami, and due to the information of the Kikōō, the Undouka brought the Sōkyoku with the expedition.
The Jishija Skirmish
The Midnight War, also dubbed the Shinigami Massacre, was a series of brutal conflicts perpetrated by Kotaro Itadori, and saw the brief return of Ashira the Deserter. The Midnight War began approximately one hundred and twenty years before Aizen's betrayal, and was riddled with guerrilla tactics. The goals of the Midnight War was to cripple the Gotei 13 by drawing out and slaying its Captains, and to lure the attention of the Spirit King. The true objective of the Midnight War was never revealed, though it was implied to be a tactical usurpation of the Spirit King's reign. The Midnight War officially began with the Ambush at Hokutan, and ended with the Nagaki Retaliation, specifically the Battle of Gengetsu Saidan.
Ambush at Hokutan
The Ambush at Hokutan was the first battle of the Midnight War, and was orchestrated by Kotaro Itadori. Undouka forces attacked a special assembly of Shinigami who'd gathered to the center of the Rukongai district Hokutan. The battle was extremely swift, lasting less than two hours, and resulted in the death of all Shinigami present, including one Captain, and approximately twelve percent of Hokutan's population. No Undouka deaths occurred.
The Zaraki Massacre
In response to the Ambush at Hokutan, the Gotei 13 organized a task force to investigate the cause of the deaths. Strangely, there were no traces of spiritual power present at the scene, even residual reiatsu. It'd all disappeared. The task force did find a single element of reiatsu, and traced it to the Zaraki district. Minutes after entering, the Shinigami task force was attacked and all were slain within moments. As with the last time, no Undouka fell.
Skirmish at Ogato
Now especially cautious, the Gotei 13 devised a special force led by the Fifth Division's Captain. The Shinigami eventually followed a trail to the Ogato district, a desert land where there was an abundant of lawless bandits and thieves. Unlike the previous occasions, the Shinigami discovered the Undouka by surprise, and killed one of the members. The action then erupted into a skirmish between both sides, until it finally ended with all Shinigami slain, including the Fifth Division Captain. Although one Undouka died before the conflict began, during the skirmish, no Undouka fell, as was the case before.
Response from above
The news of the "Shinigami massacre" had spread far beyond the Gotei 13, to the extent that even the Central 46 Chambers could not contain the incidents. The response invoked fear in many, and much respect toward the Shinigami was lost. To quickly end the mysterious attacks on the Shinigami, two guards of the Ring of Two from the Royal Special Task Force were eventually called upon and dispatched to aid the Gotei 13.
Battle of Kusajishi
After a month of investigation, the two Royal Guards eventually discovered a location in Kusajishi with an abnormal amount of activity. Upon arrival, a large Undouka campsite was discovered. Before the Shinigami could retreat, they were discovered, and the Battle of Kusajishi began. The battle became legendary, and the most famous of the Undouka Wars, which ended with the death of two Royal Guards, two Shinigami Captains, and the nearly the entire Shinigami force, save for one lieutenant who died soon after returning to the Seireitei. On the other side, only three Undouka out of the entire force died.
Raid on the Spirit King's patrol
At some point after the Battle of Kusajishi, the Undouka planned a raid on the Spirit King's royal patrols, who had to descend into Soul Society to travel to a different point in Heaven. During their descent, there was an especially large concentration of guards. Assuming it to be a precaution from the earlier attacks, the Undouka attacked the guard. During the raid, one of the guards escaped with a small boy, falling to the Inuzuri district. The Royal Guard soon died afterward, as did the patrol, while the Undouka suffered no casualties. However, Kotaro discovered the identity of the young boy who apparently escaped, and quickly organized a manhunt for him, believing his capture would lure the Spirit King's notice greater than any other affliction on the Gotei 13 would.
Battle in the heart of Inuzuri: Ashira vs Bukai
Kotaro dispatched two of his most elite warriors to hunt down the young boy who escaped from the raid. The two were Bukai, Kotaro's second in command, and Uraki, Bukai's younger brother. During the search in Inuzuri, the two brothers were to search from either ends of Inuzuri, and regroup in the district's center. On the third day of the search, Bukai grew impatient, and once he reached the center of the Inuzuri district, continued his search on his own. In a stroke of chance, Bukai noticed a bounty hunter carrying a small wanted poster for a renegade Shinigami. After following him for only a short while, Bukai felt a unique, almost majestic, reiatsu. Decided to take matters inot his own hands, Bukai killed the bounty hunter, and fought against an individual who he believed was either a hired guard or the renegade. In the end, Bukai was slain, though he was found by Uraki. When the report came back to Kotaro, he launched a full-scale raid in Inuzuri's surrounding districts. With the exception of the boy, all who saw them were to be slain.
Capture of Kiryoku
During the second day of Kotaro's raid, some 12,000 residents of Inuzuri, and several of its neighboring districts, such as Nagaki, had been murdered, with the body count rapidly rising. During the day, the young boy being searched for, Kiryoku, was discovered in the mountainous region of Nagaki. Wasting no time with his capture, Kiryoku was captured, though was pursued by Ashira, who was perceived to be an individual in ragged clothing and a sword. Though Ashira was unable to catch up to Kiryoku's kidnappers due top his inexperience in regularly using his reiatsu skills, he was intercepted by a massive task force of Rukongai warriors known as the Adauchi. Although initially seen as an enemy, Ashira was able to reason with the Adauchi's leader, who was an old acquaintance of his. After being informed of the "Red Warrior's" hideout, Ashira led the Adauchi into the Battle of Gengetsu Saidan.
The Nagaki Retaliation: Battle of Gengetsu Saidan
The Nagaki Retaliation was swift, and the attack came from the North and South simultaneously. During the early stages of the battle, the Adauchi gained a brief advantage over the "Red Warriors", though it was short lived. The entire district was suddenly engulfed in a mysterious spell that seemed to not only negate, but totally remove their spiritual power and traits as a soul, effectively rendering them, for all intents and purposes, human-like. Although both sides were adept in fighting without the use of spiritual techniques, the "Red Warriors" proved far superior to the Adauchi, being weaker only in numbers. The Battle of Gengetsu Saidan lasted almost ten hours, with both sides nearly exhausted, both in stamina and manpower. During the chaos, Ashira, injured from battle, climbed the altar to rescue Kiryoku, who was held captive near its peak. Below, Kotaro was the last of his warriors fighting, and slew the remaining Adauchi. Upon seeing Ashira freeing Kiryoku, Kotaro quickly moved to their position to confront the last enemy.
Duel on the Gengetsu Saidan
The Duel on the Gengetsu Saidan was the "final" confrontation of the Midnight War. After Kiryoku had been freed, he and Ashira were met by the last surviving "Red Warrior", Kotaro Itadori himself. Main article: Duel on the Gengetsu Saidan